Schedule III - Real Estate and Accumulated Depreciation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	$ 655,934
Initial Cost [Abstract]
Land	625,594
Buildings and Improvements	3,362,277
Net Improvements (Retirements) Since Acquisition	643,257
Carrying Amount at Which Carried [Abstract]
Land	673,838
Buildings and Improvements	3,957,290
Total	4,631,128	4,669,289	4,726,169	4,793,080
Accumulated Depreciation at December 31, 2013	1,078,996	983,808	954,665	865,710
400 Berwyn Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,657
Buildings and Improvements	4,462
Net Improvements (Retirements) Since Acquisition	14,011
Carrying Amount at Which Carried [Abstract]
Land	2,657
Buildings and Improvements	18,473
Total	21,130
Accumulated Depreciation at December 31, 2013	6,255
Depreciable Life (Years)	40 years
300 Berwyn Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,206
Buildings and Improvements	13,422
Net Improvements (Retirements) Since Acquisition	3,800
Carrying Amount at Which Carried [Abstract]
Land	2,206
Buildings and Improvements	17,222
Total	19,428
Accumulated Depreciation at December 31, 2013	8,280
Depreciable Life (Years)	40 years
1050 Westlakes Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,611
Buildings and Improvements	10,445
Net Improvements (Retirements) Since Acquisition	5,047
Carrying Amount at Which Carried [Abstract]
Land	2,611
Buildings and Improvements	15,492
Total	18,103
Accumulated Depreciation at December 31, 2013	7,984
Depreciable Life (Years)	40 years
1200 Swedesford Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,595
Buildings and Improvements	11,809
Net Improvements (Retirements) Since Acquisition	3,534
Carrying Amount at Which Carried [Abstract]
Land	2,595
Buildings and Improvements	15,343
Total	17,938
Accumulated Depreciation at December 31, 2013	6,457
Depreciable Life (Years)	40 years
200 Berwyn Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,533
Buildings and Improvements	9,460
Net Improvements (Retirements) Since Acquisition	2,466
Carrying Amount at Which Carried [Abstract]
Land	1,533
Buildings and Improvements	11,926
Total	13,459
Accumulated Depreciation at December 31, 2013	5,751
Depreciable Life (Years)	40 years
1180 Swedesford Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,086
Buildings and Improvements	8,342
Net Improvements (Retirements) Since Acquisition	2,979
Carrying Amount at Which Carried [Abstract]
Land	2,086
Buildings and Improvements	11,321
Total	13,407
Accumulated Depreciation at December 31, 2013	3,441
Depreciable Life (Years)	40 years
100 Berwyn Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,180
Buildings and Improvements	7,290
Net Improvements (Retirements) Since Acquisition	2,228
Carrying Amount at Which Carried [Abstract]
Land	1,180
Buildings and Improvements	9,518
Total	10,698
Accumulated Depreciation at December 31, 2013	4,661
Depreciable Life (Years)	40 years
1160 Swedesford Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,781
Buildings and Improvements	7,124
Net Improvements (Retirements) Since Acquisition	4,255
Carrying Amount at Which Carried [Abstract]
Land	2,045
Buildings and Improvements	11,115
Total	13,160
Accumulated Depreciation at December 31, 2013	2,961
Depreciable Life (Years)	40 years
1100 Cassett Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,695
Buildings and Improvements	6,779
Net Improvements (Retirements) Since Acquisition	1,269
Carrying Amount at Which Carried [Abstract]
Land	1,695
Buildings and Improvements	8,048
Total	9,743
Accumulated Depreciation at December 31, 2013	2,561
Depreciable Life (Years)	40 years
980 Harvest Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,079
Buildings and Improvements	8,315
Net Improvements (Retirements) Since Acquisition	736
Carrying Amount at Which Carried [Abstract]
Land	2,079
Buildings and Improvements	9,051
Total	11,130
Accumulated Depreciation at December 31, 2013	2,935
Depreciable Life (Years)	40 years
910 Harvest Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,224
Buildings and Improvements	8,645
Net Improvements (Retirements) Since Acquisition	(730)
Carrying Amount at Which Carried [Abstract]
Land	1,224
Buildings and Improvements	7,915
Total	9,139
Accumulated Depreciation at December 31, 2013	4,394
Depreciable Life (Years)	40 years
925 Harvest Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,671
Buildings and Improvements	6,606
Net Improvements (Retirements) Since Acquisition	993
Carrying Amount at Which Carried [Abstract]
Land	1,671
Buildings and Improvements	7,599
Total	9,270
Accumulated Depreciation at December 31, 2013	3,480
Depreciable Life (Years)	40 years
920 Harvest Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,209
Buildings and Improvements	6,595
Net Improvements (Retirements) Since Acquisition	(684)
Carrying Amount at Which Carried [Abstract]
Land	1,209
Buildings and Improvements	5,911
Total	7,120
Accumulated Depreciation at December 31, 2013	3,134
Depreciable Life (Years)	40 years
Six Tower Bridge [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,927
Buildings and Improvements	14,722
Net Improvements (Retirements) Since Acquisition	786
Carrying Amount at Which Carried [Abstract]
Land	6,927
Buildings and Improvements	15,508
Total	22,435
Accumulated Depreciation at December 31, 2013	698
Depreciable Life (Years)	40 years
426 Lancaster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,689
Buildings and Improvements	6,756
Net Improvements (Retirements) Since Acquisition	376
Carrying Amount at Which Carried [Abstract]
Land	1,689
Buildings and Improvements	7,132
Total	8,821
Accumulated Depreciation at December 31, 2013	3,431
Depreciable Life (Years)	40 years
52 Swedesford Square [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,241
Buildings and Improvements	16,579
Net Improvements (Retirements) Since Acquisition	3,262
Carrying Amount at Which Carried [Abstract]
Land	4,241
Buildings and Improvements	19,841
Total	24,082
Accumulated Depreciation at December 31, 2013	8,392
Depreciable Life (Years)	40 years
One Progress Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,399
Buildings and Improvements	5,629
Net Improvements (Retirements) Since Acquisition	5,202
Carrying Amount at Which Carried [Abstract]
Land	1,399
Buildings and Improvements	10,831
Total	12,230
Accumulated Depreciation at December 31, 2013	4,457
Depreciable Life (Years)	40 years
500 Enterprise Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,303
Buildings and Improvements	5,188
Net Improvements (Retirements) Since Acquisition	3,743
Carrying Amount at Which Carried [Abstract]
Land	1,303
Buildings and Improvements	8,931
Total	10,234
Accumulated Depreciation at December 31, 2013	4,098
Depreciable Life (Years)	40 years
640 Freedom Business Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,222
Buildings and Improvements	16,891
Net Improvements (Retirements) Since Acquisition	4,686
Carrying Amount at Which Carried [Abstract]
Land	4,222
Buildings and Improvements	21,577
Total	25,799
Accumulated Depreciation at December 31, 2013	9,535
Depreciable Life (Years)	40 years
555 Croton Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,486
Buildings and Improvements	17,943
Net Improvements (Retirements) Since Acquisition	1,412
Carrying Amount at Which Carried [Abstract]
Land	4,486
Buildings and Improvements	19,355
Total	23,841
Accumulated Depreciation at December 31, 2013	6,779
Depreciable Life (Years)	40 years
630 Allendale Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,836
Buildings and Improvements	4,028
Net Improvements (Retirements) Since Acquisition	12,484
Carrying Amount at Which Carried [Abstract]
Land	2,898
Buildings and Improvements	16,450
Total	19,348
Accumulated Depreciation at December 31, 2013	5,778
Depreciable Life (Years)	40 years
620 Freedom Business Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,770
Buildings and Improvements	11,014
Net Improvements (Retirements) Since Acquisition	3,375
Carrying Amount at Which Carried [Abstract]
Land	2,770
Buildings and Improvements	14,389
Total	17,159
Accumulated Depreciation at December 31, 2013	7,515
Depreciable Life (Years)	40 years
1000 First Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,772
Buildings and Improvements	10,936
Net Improvements (Retirements) Since Acquisition	2,117
Carrying Amount at Which Carried [Abstract]
Land	2,772
Buildings and Improvements	13,053
Total	15,825
Accumulated Depreciation at December 31, 2013	5,894
Depreciable Life (Years)	40 years
1060 First Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,712
Buildings and Improvements	10,953
Net Improvements (Retirements) Since Acquisition	4,522
Carrying Amount at Which Carried [Abstract]
Land	2,712
Buildings and Improvements	15,475
Total	18,187
Accumulated Depreciation at December 31, 2013	6,646
Depreciable Life (Years)	40 years
630 Freedom Business Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,773
Buildings and Improvements	11,144
Net Improvements (Retirements) Since Acquisition	2,343
Carrying Amount at Which Carried [Abstract]
Land	2,773
Buildings and Improvements	13,487
Total	16,260
Accumulated Depreciation at December 31, 2013	6,300
Depreciable Life (Years)	40 years
1040 First Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,860
Buildings and Improvements	11,282
Net Improvements (Retirements) Since Acquisition	6,080
Carrying Amount at Which Carried [Abstract]
Land	2,860
Buildings and Improvements	17,362
Total	20,222
Accumulated Depreciation at December 31, 2013	5,460
Depreciable Life (Years)	40 years
1020 First Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,168
Buildings and Improvements	8,576
Net Improvements (Retirements) Since Acquisition	7,617
Carrying Amount at Which Carried [Abstract]
Land	2,168
Buildings and Improvements	16,193
Total	18,361
Accumulated Depreciation at December 31, 2013	6,766
Depreciable Life (Years)	40 years
610 Freedom Business Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,017
Buildings and Improvements	8,070
Net Improvements (Retirements) Since Acquisition	2,393
Carrying Amount at Which Carried [Abstract]
Land	2,017
Buildings and Improvements	10,463
Total	12,480
Accumulated Depreciation at December 31, 2013	4,883
Depreciable Life (Years)	40 years
650 Park Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,916
Buildings and Improvements	4,378
Net Improvements (Retirements) Since Acquisition	1,701
Carrying Amount at Which Carried [Abstract]
Land	1,916
Buildings and Improvements	6,079
Total	7,995
Accumulated Depreciation at December 31, 2013	2,920
Depreciable Life (Years)	40 years
500 North Gulph Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,303
Buildings and Improvements	5,201
Net Improvements (Retirements) Since Acquisition	1,522
Carrying Amount at Which Carried [Abstract]
Land	1,303
Buildings and Improvements	6,723
Total	8,026
Accumulated Depreciation at December 31, 2013	3,390
Depreciable Life (Years)	40 years
741 First Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,287
Buildings and Improvements	5,151
Net Improvements (Retirements) Since Acquisition	11
Carrying Amount at Which Carried [Abstract]
Land	1,287
Buildings and Improvements	5,162
Total	6,449
Accumulated Depreciation at December 31, 2013	2,363
Depreciable Life (Years)	40 years
751-761 Fifth Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,097
Buildings and Improvements	4,391
Net Improvements (Retirements) Since Acquisition	31
Carrying Amount at Which Carried [Abstract]
Land	1,097
Buildings and Improvements	4,422
Total	5,519
Accumulated Depreciation at December 31, 2013	2,021
Depreciable Life (Years)	40 years
600 Park Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,012
Buildings and Improvements	4,048
Net Improvements (Retirements) Since Acquisition	385
Carrying Amount at Which Carried [Abstract]
Land	1,012
Buildings and Improvements	4,433
Total	5,445
Accumulated Depreciation at December 31, 2013	2,045
Depreciable Life (Years)	40 years
620 Allendale Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,020
Buildings and Improvements	3,839
Net Improvements (Retirements) Since Acquisition	658
Carrying Amount at Which Carried [Abstract]
Land	1,020
Buildings and Improvements	4,497
Total	5,517
Accumulated Depreciation at December 31, 2013	2,008
Depreciable Life (Years)	40 years
660 Allendale Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	396
Buildings and Improvements	948
Net Improvements (Retirements) Since Acquisition	2,957
Carrying Amount at Which Carried [Abstract]
Land	1,085
Buildings and Improvements	3,216
Total	4,301
Accumulated Depreciation at December 31, 2013	921
Depreciable Life (Years)	40 years
640 Allendale Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	439
Buildings and Improvements	432
Net Improvements (Retirements) Since Acquisition	1,580
Carrying Amount at Which Carried [Abstract]
Land	439
Buildings and Improvements	2,012
Total	2,451
Accumulated Depreciation at December 31, 2013	802
Depreciable Life (Years)	40 years
14 Campus Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,244
Buildings and Improvements	4,217
Net Improvements (Retirements) Since Acquisition	1,533
Carrying Amount at Which Carried [Abstract]
Land	2,244
Buildings and Improvements	5,750
Total	7,994
Accumulated Depreciation at December 31, 2013	3,322
Depreciable Life (Years)	40 years
11 Campus Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,112
Buildings and Improvements	4,067
Net Improvements (Retirements) Since Acquisition	1,006
Carrying Amount at Which Carried [Abstract]
Land	1,112
Buildings and Improvements	5,073
Total	6,185
Accumulated Depreciation at December 31, 2013	1,967
Depreciable Life (Years)	40 years
17 Campus Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,108
Buildings and Improvements	5,155
Net Improvements (Retirements) Since Acquisition	(374)
Carrying Amount at Which Carried [Abstract]
Land	1,108
Buildings and Improvements	4,781
Total	5,889
Accumulated Depreciation at December 31, 2013	1,728
Depreciable Life (Years)	40 years
15 Campus Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,164
Buildings and Improvements	3,896
Net Improvements (Retirements) Since Acquisition	249
Carrying Amount at Which Carried [Abstract]
Land	1,164
Buildings and Improvements	4,145
Total	5,309
Accumulated Depreciation at December 31, 2013	1,394
Depreciable Life (Years)	40 years
18 Campus Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	787
Buildings and Improvements	3,312
Net Improvements (Retirements) Since Acquisition	(77)
Carrying Amount at Which Carried [Abstract]
Land	787
Buildings and Improvements	3,235
Total	4,022
Accumulated Depreciation at December 31, 2013	1,631
Depreciable Life (Years)	40 years
401 Plymouth Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,198
Buildings and Improvements	16,131
Net Improvements (Retirements) Since Acquisition	16,130
Carrying Amount at Which Carried [Abstract]
Land	6,198
Buildings and Improvements	32,261
Total	38,459
Accumulated Depreciation at December 31, 2013	9,982
Depreciable Life (Years)	40 years
4000 Chemical Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,373
Buildings and Improvements	24,546
Net Improvements (Retirements) Since Acquisition	1,467
Carrying Amount at Which Carried [Abstract]
Land	4,373
Buildings and Improvements	26,013
Total	30,386
Accumulated Depreciation at December 31, 2013	4,864
Depreciable Life (Years)	40 years
610 West Germantown Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,651
Buildings and Improvements	14,514
Net Improvements (Retirements) Since Acquisition	3,357
Carrying Amount at Which Carried [Abstract]
Land	3,651
Buildings and Improvements	17,871
Total	21,522
Accumulated Depreciation at December 31, 2013	5,892
Depreciable Life (Years)	40 years
600 West Germantown Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,652
Buildings and Improvements	15,288
Net Improvements (Retirements) Since Acquisition	1,609
Carrying Amount at Which Carried [Abstract]
Land	3,652
Buildings and Improvements	16,897
Total	20,549
Accumulated Depreciation at December 31, 2013	5,659
Depreciable Life (Years)	40 years
630 West Germantown Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,558
Buildings and Improvements	14,743
Net Improvements (Retirements) Since Acquisition	1,902
Carrying Amount at Which Carried [Abstract]
Land	3,558
Buildings and Improvements	16,645
Total	20,203
Accumulated Depreciation at December 31, 2013	5,994
Depreciable Life (Years)	40 years
620 West Germantown Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,572
Buildings and Improvements	14,435
Net Improvements (Retirements) Since Acquisition	1,339
Carrying Amount at Which Carried [Abstract]
Land	3,572
Buildings and Improvements	15,774
Total	19,346
Accumulated Depreciation at December 31, 2013	5,378
Depreciable Life (Years)	40 years
660 West Germantown Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,694
Buildings and Improvements	5,487
Net Improvements (Retirements) Since Acquisition	18,484
Carrying Amount at Which Carried [Abstract]
Land	5,404
Buildings and Improvements	22,261
Total	27,665
Accumulated Depreciation at December 31, 2013	1,436
Depreciable Life (Years)	30 years
2240/2250 Butler Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,104
Buildings and Improvements	4,627
Net Improvements (Retirements) Since Acquisition	1,312
Carrying Amount at Which Carried [Abstract]
Land	1,104
Buildings and Improvements	5,939
Total	7,043
Accumulated Depreciation at December 31, 2013	3,596
Depreciable Life (Years)	40 years
2260 Butler Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	661
Buildings and Improvements	2,727
Net Improvements (Retirements) Since Acquisition	2,115
Carrying Amount at Which Carried [Abstract]
Land	661
Buildings and Improvements	4,842
Total	5,503
Accumulated Depreciation at December 31, 2013	2,165
Depreciable Life (Years)	40 years
120 West Germantown Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	685
Buildings and Improvements	2,773
Net Improvements (Retirements) Since Acquisition	729
Carrying Amount at Which Carried [Abstract]
Land	685
Buildings and Improvements	3,502
Total	4,187
Accumulated Depreciation at December 31, 2013	1,767
Depreciable Life (Years)	40 years
140 West Germantown Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	481
Buildings and Improvements	1,976
Net Improvements (Retirements) Since Acquisition	282
Carrying Amount at Which Carried [Abstract]
Land	481
Buildings and Improvements	2,258
Total	2,739
Accumulated Depreciation at December 31, 2013	1,079
Depreciable Life (Years)	40 years
351 Plymouth Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,043
Buildings and Improvements	555
Net Improvements (Retirements) Since Acquisition	0
Carrying Amount at Which Carried [Abstract]
Land	1,043
Buildings and Improvements	555
Total	1,598
Accumulated Depreciation at December 31, 2013	135
Depreciable Life (Years)	40 years
150 Radnor Chester Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	11,925
Buildings and Improvements	36,986
Net Improvements (Retirements) Since Acquisition	12,549
Carrying Amount at Which Carried [Abstract]
Land	11,897
Buildings and Improvements	49,563
Total	61,460
Accumulated Depreciation at December 31, 2013	16,431
Depreciable Life (Years)	29 years
One Radnor Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	7,323
Buildings and Improvements	28,613
Net Improvements (Retirements) Since Acquisition	23,182
Carrying Amount at Which Carried [Abstract]
Land	7,323
Buildings and Improvements	51,795
Total	59,118
Accumulated Depreciation at December 31, 2013	18,406
Depreciable Life (Years)	29 years
201 King of Prussia Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	8,956
Buildings and Improvements	29,811
Net Improvements (Retirements) Since Acquisition	5,070
Carrying Amount at Which Carried [Abstract]
Land	8,949
Buildings and Improvements	34,888
Total	43,837
Accumulated Depreciation at December 31, 2013	15,445
Depreciable Life (Years)	25 years
555 Lancaster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	8,014
Buildings and Improvements	16,508
Net Improvements (Retirements) Since Acquisition	16,854
Carrying Amount at Which Carried [Abstract]
Land	8,609
Buildings and Improvements	32,767
Total	41,376
Accumulated Depreciation at December 31, 2013	12,563
Depreciable Life (Years)	24 years
Four Radnor Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	5,406
Buildings and Improvements	21,390
Net Improvements (Retirements) Since Acquisition	8,501
Carrying Amount at Which Carried [Abstract]
Land	5,705
Buildings and Improvements	29,592
Total	35,297
Accumulated Depreciation at December 31, 2013	10,480
Depreciable Life (Years)	30 years
Five Radnor Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,506
Buildings and Improvements	25,525
Net Improvements (Retirements) Since Acquisition	5,455
Carrying Amount at Which Carried [Abstract]
Land	6,578
Buildings and Improvements	30,908
Total	37,486
Accumulated Depreciation at December 31, 2013	8,024
Depreciable Life (Years)	38 years
Three Radnor Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,773
Buildings and Improvements	17,961
Net Improvements (Retirements) Since Acquisition	2,645
Carrying Amount at Which Carried [Abstract]
Land	4,791
Buildings and Improvements	20,588
Total	25,379
Accumulated Depreciation at December 31, 2013	8,094
Depreciable Life (Years)	29 years
Two Radnor Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,937
Buildings and Improvements	15,484
Net Improvements (Retirements) Since Acquisition	3,888
Carrying Amount at Which Carried [Abstract]
Land	3,942
Buildings and Improvements	19,367
Total	23,309
Accumulated Depreciation at December 31, 2013	6,882
Depreciable Life (Years)	29 years
130 Radnor Chester Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,573
Buildings and Improvements	8,338
Net Improvements (Retirements) Since Acquisition	3,483
Carrying Amount at Which Carried [Abstract]
Land	2,567
Buildings and Improvements	11,827
Total	14,394
Accumulated Depreciation at December 31, 2013	4,357
Depreciable Life (Years)	25 years
170 Radnor Chester Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,514
Buildings and Improvements	8,147
Net Improvements (Retirements) Since Acquisition	1,718
Carrying Amount at Which Carried [Abstract]
Land	2,509
Buildings and Improvements	9,870
Total	12,379
Accumulated Depreciation at December 31, 2013	3,051
Depreciable Life (Years)	25 years
200 N. Radnor Chester Rd. [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,366
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	3,675
Carrying Amount at Which Carried [Abstract]
Land	3,366
Buildings and Improvements	3,675
Total	7,041
Accumulated Depreciation at December 31, 2013	113
Depreciable Life (Years)	40 years
101 West Elm Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,251
Buildings and Improvements	25,209
Net Improvements (Retirements) Since Acquisition	3,189
Carrying Amount at Which Carried [Abstract]
Land	6,251
Buildings and Improvements	28,398
Total	34,649
Accumulated Depreciation at December 31, 2013	7,640
Depreciable Life (Years)	40 years
1 West Elm Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,557
Buildings and Improvements	14,249
Net Improvements (Retirements) Since Acquisition	3,876
Carrying Amount at Which Carried [Abstract]
Land	3,557
Buildings and Improvements	18,125
Total	21,682
Accumulated Depreciation at December 31, 2013	4,204
Depreciable Life (Years)	40 years
595 East Swedesford Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,729
Buildings and Improvements	10,917
Net Improvements (Retirements) Since Acquisition	2,307
Carrying Amount at Which Carried [Abstract]
Land	2,729
Buildings and Improvements	13,224
Total	15,953
Accumulated Depreciation at December 31, 2013	4,464
Depreciable Life (Years)	40 years
575 East Swedesford Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,178
Buildings and Improvements	8,712
Net Improvements (Retirements) Since Acquisition	1,681
Carrying Amount at Which Carried [Abstract]
Land	2,178
Buildings and Improvements	10,393
Total	12,571
Accumulated Depreciation at December 31, 2013	3,692
Depreciable Life (Years)	40 years
565 East Swedesford Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,872
Buildings and Improvements	7,489
Net Improvements (Retirements) Since Acquisition	1,744
Carrying Amount at Which Carried [Abstract]
Land	1,872
Buildings and Improvements	9,233
Total	11,105
Accumulated Depreciation at December 31, 2013	3,166
Depreciable Life (Years)	40 years
585 East Swedesford Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,350
Buildings and Improvements	5,401
Net Improvements (Retirements) Since Acquisition	1,350
Carrying Amount at Which Carried [Abstract]
Land	1,350
Buildings and Improvements	6,751
Total	8,101
Accumulated Depreciation at December 31, 2013	1,551
Depreciable Life (Years)	40 years
2970 Market Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	184,442
Initial Cost [Abstract]
Land	22,430
Buildings and Improvements	217,763
Net Improvements (Retirements) Since Acquisition	12,021
Carrying Amount at Which Carried [Abstract]
Land	22,430
Buildings and Improvements	229,784
Total	252,214
Accumulated Depreciation at December 31, 2013	27,560
Depreciable Life (Years)	40 years
2929 Arch Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	208,570
Net Improvements (Retirements) Since Acquisition	29,302
Carrying Amount at Which Carried [Abstract]
Land	12,586
Buildings and Improvements	225,286
Total	237,872
Accumulated Depreciation at December 31, 2013	85,000
Depreciable Life (Years)	40 years
1717 Arch Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	98,188
Net Improvements (Retirements) Since Acquisition	66,550
Carrying Amount at Which Carried [Abstract]
Land	25,195
Buildings and Improvements	139,543
Total	164,738
Accumulated Depreciation at December 31, 2013	19,000
Depreciable Life (Years)	40 years
2001 Market Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	112,000
Initial Cost [Abstract]
Land	15,323
Buildings and Improvements	120,842
Net Improvements (Retirements) Since Acquisition	2,125
Carrying Amount at Which Carried [Abstract]
Land	15,323
Buildings and Improvements	122,967
Total	138,290
Accumulated Depreciation at December 31, 2013	3,371
Depreciable Life (Years)	40 years
130 North 18th Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	14,496
Buildings and Improvements	107,736
Net Improvements (Retirements) Since Acquisition	17,397
Carrying Amount at Which Carried [Abstract]
Land	14,473
Buildings and Improvements	125,156
Total	139,629
Accumulated Depreciation at December 31, 2013	37,755
Depreciable Life (Years)	34 years
100 North 18th Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	87,767
Initial Cost [Abstract]
Land	16,066
Buildings and Improvements	100,255
Net Improvements (Retirements) Since Acquisition	10,925
Carrying Amount at Which Carried [Abstract]
Land	16,066
Buildings and Improvements	111,180
Total	127,246
Accumulated Depreciation at December 31, 2013	35,435
Depreciable Life (Years)	36 years
2005 Market Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	123,205
Initial Cost [Abstract]
Land	15,161
Buildings and Improvements	122,486
Net Improvements (Retirements) Since Acquisition	(11,726)
Carrying Amount at Which Carried [Abstract]
Land	15,161
Buildings and Improvements	110,760
Total	125,921
Accumulated Depreciation at December 31, 2013	2,959
Depreciable Life (Years)	40 years
2930 Chestnut Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	37,765
Initial Cost [Abstract]
Land	0
Buildings and Improvements	76,008
Net Improvements (Retirements) Since Acquisition	2,644
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	78,652
Total	78,652
Accumulated Depreciation at December 31, 2013	8,677
Depreciable Life (Years)	40 years
1900 Market [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	7,768
Buildings and Improvements	17,263
Net Improvements (Retirements) Since Acquisition	481
Carrying Amount at Which Carried [Abstract]
Land	7,768
Buildings and Improvements	17,744
Total	25,512
Accumulated Depreciation at December 31, 2013	1,246
Depreciable Life (Years)	30 years
3020 Market Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	21,417
Net Improvements (Retirements) Since Acquisition	6,225
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	27,642
Total	27,642
Accumulated Depreciation at December 31, 2013	3,260
Depreciable Life (Years)	26 years
101-103 Juniper Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	14,401
Net Improvements (Retirements) Since Acquisition	324
Carrying Amount at Which Carried [Abstract]
Land	478
Buildings and Improvements	14,247
Total	14,725
Accumulated Depreciation at December 31, 2013	1,865
Depreciable Life (Years)	40 years
Philadelphia Marine Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	532
Buildings and Improvements	2,196
Net Improvements (Retirements) Since Acquisition	3,444
Carrying Amount at Which Carried [Abstract]
Land	628
Buildings and Improvements	5,544
Total	6,172
Accumulated Depreciation at December 31, 2013	2,408
Depreciable Life (Years)	40 years
11720 Beltsville Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,831
Buildings and Improvements	16,661
Net Improvements (Retirements) Since Acquisition	3,399
Carrying Amount at Which Carried [Abstract]
Land	3,904
Buildings and Improvements	19,987
Total	23,891
Accumulated Depreciation at December 31, 2013	5,887
Depreciable Life (Years)	46 years
11700 Beltsville Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,808
Buildings and Improvements	12,081
Net Improvements (Retirements) Since Acquisition	293
Carrying Amount at Which Carried [Abstract]
Land	2,863
Buildings and Improvements	12,319
Total	15,182
Accumulated Depreciation at December 31, 2013	2,702
Depreciable Life (Years)	46 years
11710 Beltsville Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,278
Buildings and Improvements	11,100
Net Improvements (Retirements) Since Acquisition	(814)
Carrying Amount at Which Carried [Abstract]
Land	2,321
Buildings and Improvements	10,243
Total	12,564
Accumulated Depreciation at December 31, 2013	2,370
Depreciable Life (Years)	46 years
6600 Rockledge Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	37,421
Net Improvements (Retirements) Since Acquisition	11,303
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	48,724
Total	48,724
Accumulated Depreciation at December 31, 2013	15,070
Depreciable Life (Years)	50 years
11740 Beltsville Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	198
Buildings and Improvements	870
Net Improvements (Retirements) Since Acquisition	42
Carrying Amount at Which Carried [Abstract]
Land	202
Buildings and Improvements	908
Total	1,110
Accumulated Depreciation at December 31, 2013	211
Depreciable Life (Years)	46 years
12015 Lee Jackson Memorial Highway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,770
Buildings and Improvements	22,895
Net Improvements (Retirements) Since Acquisition	3,987
Carrying Amount at Which Carried [Abstract]
Land	3,841
Buildings and Improvements	26,811
Total	30,652
Accumulated Depreciation at December 31, 2013	7,188
Depreciable Life (Years)	42 years
11781 Lee Jackson Memorial Highway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,246
Buildings and Improvements	19,836
Net Improvements (Retirements) Since Acquisition	567
Carrying Amount at Which Carried [Abstract]
Land	3,307
Buildings and Improvements	20,342
Total	23,649
Accumulated Depreciation at December 31, 2013	4,748
Depreciable Life (Years)	40 years
4401 Fair Lakes Court [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,569
Buildings and Improvements	11,982
Net Improvements (Retirements) Since Acquisition	247
Carrying Amount at Which Carried [Abstract]
Land	1,599
Buildings and Improvements	12,199
Total	13,798
Accumulated Depreciation at December 31, 2013	2,456
Depreciable Life (Years)	52 years
3141 Fairview Park Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	21,242
Initial Cost [Abstract]
Land	5,918
Buildings and Improvements	40,981
Net Improvements (Retirements) Since Acquisition	12,017
Carrying Amount at Which Carried [Abstract]
Land	7,080
Buildings and Improvements	51,836
Total	58,916
Accumulated Depreciation at December 31, 2013	10,264
Depreciable Life (Years)	51 years
2340 Dulles Corner Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	16,345
Buildings and Improvements	65,379
Net Improvements (Retirements) Since Acquisition	18,280
Carrying Amount at Which Carried [Abstract]
Land	16,129
Buildings and Improvements	83,875
Total	100,004
Accumulated Depreciation at December 31, 2013	28,064
Depreciable Life (Years)	40 years
2291 Wood Oak Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	8,243
Buildings and Improvements	52,413
Net Improvements (Retirements) Since Acquisition	13,499
Carrying Amount at Which Carried [Abstract]
Land	8,782
Buildings and Improvements	65,373
Total	74,155
Accumulated Depreciation at December 31, 2013	13,524
Depreciable Life (Years)	55 years
196/198 Van Buren Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	7,931
Buildings and Improvements	43,812
Net Improvements (Retirements) Since Acquisition	9,040
Carrying Amount at Which Carried [Abstract]
Land	8,348
Buildings and Improvements	52,435
Total	60,783
Accumulated Depreciation at December 31, 2013	10,308
Depreciable Life (Years)	53 years
2251 Corporate Park Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	11,472
Buildings and Improvements	45,893
Net Improvements (Retirements) Since Acquisition	2,276
Carrying Amount at Which Carried [Abstract]
Land	11,472
Buildings and Improvements	48,169
Total	59,641
Accumulated Depreciation at December 31, 2013	9,758
Depreciable Life (Years)	40 years
2355 Dulles Corner Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	10,365
Buildings and Improvements	43,876
Net Improvements (Retirements) Since Acquisition	4,648
Carrying Amount at Which Carried [Abstract]
Land	10,365
Buildings and Improvements	48,524
Total	58,889
Accumulated Depreciation at December 31, 2013	12,675
Depreciable Life (Years)	40 years
2411 Dulles Corner Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	7,279
Buildings and Improvements	46,340
Net Improvements (Retirements) Since Acquisition	117
Carrying Amount at Which Carried [Abstract]
Land	7,417
Buildings and Improvements	46,319
Total	53,736
Accumulated Depreciation at December 31, 2013	8,645
Depreciable Life (Years)	50 years
13880 Dulles Corner Lane [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	7,236
Buildings and Improvements	39,213
Net Improvements (Retirements) Since Acquisition	3,369
Carrying Amount at Which Carried [Abstract]
Land	7,373
Buildings and Improvements	42,445
Total	49,818
Accumulated Depreciation at December 31, 2013	7,824
Depreciable Life (Years)	55 years
2121 Cooperative Way [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	5,598
Buildings and Improvements	38,639
Net Improvements (Retirements) Since Acquisition	2,532
Carrying Amount at Which Carried [Abstract]
Land	5,795
Buildings and Improvements	40,974
Total	46,769
Accumulated Depreciation at December 31, 2013	7,665
Depreciable Life (Years)	54 years
2201 Cooperative Way [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,809
Buildings and Improvements	34,093
Net Improvements (Retirements) Since Acquisition	4,513
Carrying Amount at Which Carried [Abstract]
Land	4,809
Buildings and Improvements	38,606
Total	43,415
Accumulated Depreciation at December 31, 2013	7,153
Depreciable Life (Years)	54 years
13825 Sunrise Valley Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,794
Buildings and Improvements	19,365
Net Improvements (Retirements) Since Acquisition	2,121
Carrying Amount at Which Carried [Abstract]
Land	3,866
Buildings and Improvements	21,414
Total	25,280
Accumulated Depreciation at December 31, 2013	4,441
Depreciable Life (Years)	46 years
1676 International Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	58,470
Initial Cost [Abstract]
Land	18,437
Buildings and Improvements	97,538
Net Improvements (Retirements) Since Acquisition	3,183
Carrying Amount at Which Carried [Abstract]
Land	18,785
Buildings and Improvements	100,373
Total	119,158
Accumulated Depreciation at December 31, 2013	16,619
Depreciable Life (Years)	55 years
8260 Greensboro Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	31,043
Initial Cost [Abstract]
Land	7,952
Buildings and Improvements	33,964
Net Improvements (Retirements) Since Acquisition	1,641
Carrying Amount at Which Carried [Abstract]
Land	8,102
Buildings and Improvements	35,455
Total	43,557
Accumulated Depreciation at December 31, 2013	7,320
Depreciable Life (Years)	52 years
2273 Research Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	5,167
Buildings and Improvements	31,110
Net Improvements (Retirements) Since Acquisition	3,764
Carrying Amount at Which Carried [Abstract]
Land	5,237
Buildings and Improvements	34,804
Total	40,041
Accumulated Depreciation at December 31, 2013	10,191
Depreciable Life (Years)	45 years
2275 Research Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	5,059
Buildings and Improvements	29,668
Net Improvements (Retirements) Since Acquisition	6,479
Carrying Amount at Which Carried [Abstract]
Land	5,154
Buildings and Improvements	36,052
Total	41,206
Accumulated Depreciation at December 31, 2013	7,672
Depreciable Life (Years)	45 years
2277 Research Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,649
Buildings and Improvements	26,952
Net Improvements (Retirements) Since Acquisition	695
Carrying Amount at Which Carried [Abstract]
Land	4,733
Buildings and Improvements	27,563
Total	32,296
Accumulated Depreciation at December 31, 2013	5,724
Depreciable Life (Years)	45 years
1900 Gallows Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	7,797
Buildings and Improvements	47,817
Net Improvements (Retirements) Since Acquisition	10,849
Carrying Amount at Which Carried [Abstract]
Land	7,944
Buildings and Improvements	58,519
Total	66,463
Accumulated Depreciation at December 31, 2013	11,008
Depreciable Life (Years)	52 years
8521 Leesburg Pike [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,316
Buildings and Improvements	30,885
Net Improvements (Retirements) Since Acquisition	4,858
Carrying Amount at Which Carried [Abstract]
Land	4,397
Buildings and Improvements	35,662
Total	40,059
Accumulated Depreciation at December 31, 2013	6,399
Depreciable Life (Years)	51 years
457 Haddonfield Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,142
Buildings and Improvements	9,120
Net Improvements (Retirements) Since Acquisition	1,402
Carrying Amount at Which Carried [Abstract]
Land	2,142
Buildings and Improvements	10,522
Total	12,664
Accumulated Depreciation at December 31, 2013	5,273
Depreciable Life (Years)	40 years
200 Lake Drive East [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,069
Buildings and Improvements	8,275
Net Improvements (Retirements) Since Acquisition	1,328
Carrying Amount at Which Carried [Abstract]
Land	2,069
Buildings and Improvements	9,603
Total	11,672
Accumulated Depreciation at December 31, 2013	3,635
Depreciable Life (Years)	40 years
220 Lake Drive East [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,144
Buildings and Improvements	8,798
Net Improvements (Retirements) Since Acquisition	825
Carrying Amount at Which Carried [Abstract]
Land	2,144
Buildings and Improvements	9,623
Total	11,767
Accumulated Depreciation at December 31, 2013	3,490
Depreciable Life (Years)	40 years
210 Lake Drive East [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,645
Buildings and Improvements	6,579
Net Improvements (Retirements) Since Acquisition	1,917
Carrying Amount at Which Carried [Abstract]
Land	1,645
Buildings and Improvements	8,496
Total	10,141
Accumulated Depreciation at December 31, 2013	2,812
Depreciable Life (Years)	40 years
20 East Clementon Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	769
Buildings and Improvements	3,055
Net Improvements (Retirements) Since Acquisition	480
Carrying Amount at Which Carried [Abstract]
Land	769
Buildings and Improvements	3,535
Total	4,304
Accumulated Depreciation at December 31, 2013	1,682
Depreciable Life (Years)	40 years
10 Foster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	244
Buildings and Improvements	971
Net Improvements (Retirements) Since Acquisition	78
Carrying Amount at Which Carried [Abstract]
Land	244
Buildings and Improvements	1,049
Total	1,293
Accumulated Depreciation at December 31, 2013	497
Depreciable Life (Years)	40 years
7 Foster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	231
Buildings and Improvements	921
Net Improvements (Retirements) Since Acquisition	123
Carrying Amount at Which Carried [Abstract]
Land	231
Buildings and Improvements	1,044
Total	1,275
Accumulated Depreciation at December 31, 2013	475
Depreciable Life (Years)	40 years
50 East Clementon Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	114
Buildings and Improvements	964
Net Improvements (Retirements) Since Acquisition	653
Carrying Amount at Which Carried [Abstract]
Land	114
Buildings and Improvements	1,617
Total	1,731
Accumulated Depreciation at December 31, 2013	483
Depreciable Life (Years)	40 years
2 Foster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	185
Buildings and Improvements	730
Net Improvements (Retirements) Since Acquisition	16
Carrying Amount at Which Carried [Abstract]
Land	185
Buildings and Improvements	746
Total	931
Accumulated Depreciation at December 31, 2013	351
Depreciable Life (Years)	40 years
4 Foster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	183
Buildings and Improvements	726
Net Improvements (Retirements) Since Acquisition	16
Carrying Amount at Which Carried [Abstract]
Land	183
Buildings and Improvements	742
Total	925
Accumulated Depreciation at December 31, 2013	343
Depreciable Life (Years)	40 years
1 Foster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	93
Buildings and Improvements	364
Net Improvements (Retirements) Since Acquisition	57
Carrying Amount at Which Carried [Abstract]
Land	93
Buildings and Improvements	421
Total	514
Accumulated Depreciation at December 31, 2013	215
Depreciable Life (Years)	40 years
5 U.S. Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	21
Buildings and Improvements	81
Net Improvements (Retirements) Since Acquisition	3
Carrying Amount at Which Carried [Abstract]
Land	21
Buildings and Improvements	84
Total	105
Accumulated Depreciation at December 31, 2013	39
Depreciable Life (Years)	40 years
5 Foster Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	9
Buildings and Improvements	32
Net Improvements (Retirements) Since Acquisition	26
Carrying Amount at Which Carried [Abstract]
Land	9
Buildings and Improvements	58
Total	67
Accumulated Depreciation at December 31, 2013	28
Depreciable Life (Years)	40 years
1120 Executive Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,074
Buildings and Improvements	8,415
Net Improvements (Retirements) Since Acquisition	4,398
Carrying Amount at Which Carried [Abstract]
Land	2,074
Buildings and Improvements	12,813
Total	14,887
Accumulated Depreciation at December 31, 2013	5,143
Depreciable Life (Years)	40 years
Two Eves Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	818
Buildings and Improvements	3,461
Net Improvements (Retirements) Since Acquisition	234
Carrying Amount at Which Carried [Abstract]
Land	818
Buildings and Improvements	3,695
Total	4,513
Accumulated Depreciation at December 31, 2013	1,825
Depreciable Life (Years)	40 years
Five Eves Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	703
Buildings and Improvements	2,819
Net Improvements (Retirements) Since Acquisition	665
Carrying Amount at Which Carried [Abstract]
Land	703
Buildings and Improvements	3,484
Total	4,187
Accumulated Depreciation at December 31, 2013	1,643
Depreciable Life (Years)	40 years
Four B Eves Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	588
Buildings and Improvements	2,369
Net Improvements (Retirements) Since Acquisition	420
Carrying Amount at Which Carried [Abstract]
Land	588
Buildings and Improvements	2,789
Total	3,377
Accumulated Depreciation at December 31, 2013	1,449
Depreciable Life (Years)	40 years
Four A Eves Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	539
Buildings and Improvements	2,168
Net Improvements (Retirements) Since Acquisition	322
Carrying Amount at Which Carried [Abstract]
Land	539
Buildings and Improvements	2,490
Total	3,029
Accumulated Depreciation at December 31, 2013	1,122
Depreciable Life (Years)	40 years
308 Harper Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,643
Buildings and Improvements	6,663
Net Improvements (Retirements) Since Acquisition	739
Carrying Amount at Which Carried [Abstract]
Land	1,643
Buildings and Improvements	7,402
Total	9,045
Accumulated Depreciation at December 31, 2013	3,258
Depreciable Life (Years)	40 years
10000 Midlantic Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,206
Buildings and Improvements	12,857
Net Improvements (Retirements) Since Acquisition	4,228
Carrying Amount at Which Carried [Abstract]
Land	3,206
Buildings and Improvements	17,085
Total	20,291
Accumulated Depreciation at December 31, 2013	7,138
Depreciable Life (Years)	40 years
700 East Gate Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,569
Buildings and Improvements	14,436
Net Improvements (Retirements) Since Acquisition	2,243
Carrying Amount at Which Carried [Abstract]
Land	3,569
Buildings and Improvements	16,679
Total	20,248
Accumulated Depreciation at December 31, 2013	7,576
Depreciable Life (Years)	40 years
15000 Midlantic Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,061
Buildings and Improvements	12,254
Net Improvements (Retirements) Since Acquisition	1,619
Carrying Amount at Which Carried [Abstract]
Land	3,061
Buildings and Improvements	13,873
Total	16,934
Accumulated Depreciation at December 31, 2013	6,518
Depreciable Life (Years)	40 years
1000 Howard Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,297
Buildings and Improvements	9,288
Net Improvements (Retirements) Since Acquisition	3,922
Carrying Amount at Which Carried [Abstract]
Land	2,297
Buildings and Improvements	13,210
Total	15,507
Accumulated Depreciation at December 31, 2013	5,900
Depreciable Life (Years)	40 years
1000 Atrium Way [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,061
Buildings and Improvements	8,180
Net Improvements (Retirements) Since Acquisition	4,531
Carrying Amount at Which Carried [Abstract]
Land	2,061
Buildings and Improvements	12,711
Total	14,772
Accumulated Depreciation at December 31, 2013	5,894
Depreciable Life (Years)	40 years
2000 Midlantic Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,202
Buildings and Improvements	8,823
Net Improvements (Retirements) Since Acquisition	2,970
Carrying Amount at Which Carried [Abstract]
Land	2,202
Buildings and Improvements	11,793
Total	13,995
Accumulated Depreciation at December 31, 2013	5,349
Depreciable Life (Years)	40 years
701 East Gate Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,736
Buildings and Improvements	6,877
Net Improvements (Retirements) Since Acquisition	1,768
Carrying Amount at Which Carried [Abstract]
Land	1,736
Buildings and Improvements	8,645
Total	10,381
Accumulated Depreciation at December 31, 2013	3,715
Depreciable Life (Years)	40 years
9000 Midlantic Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,472
Buildings and Improvements	5,895
Net Improvements (Retirements) Since Acquisition	2,185
Carrying Amount at Which Carried [Abstract]
Land	1,472
Buildings and Improvements	8,080
Total	9,552
Accumulated Depreciation at December 31, 2013	3,503
Depreciable Life (Years)	40 years
307 Fellowship Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,565
Buildings and Improvements	6,342
Net Improvements (Retirements) Since Acquisition	1,517
Carrying Amount at Which Carried [Abstract]
Land	1,565
Buildings and Improvements	7,859
Total	9,424
Accumulated Depreciation at December 31, 2013	3,472
Depreciable Life (Years)	40 years
305 Fellowship Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,421
Buildings and Improvements	5,768
Net Improvements (Retirements) Since Acquisition	1,631
Carrying Amount at Which Carried [Abstract]
Land	1,421
Buildings and Improvements	7,399
Total	8,820
Accumulated Depreciation at December 31, 2013	3,122
Depreciable Life (Years)	40 years
303 Fellowship Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,493
Buildings and Improvements	6,055
Net Improvements (Retirements) Since Acquisition	851
Carrying Amount at Which Carried [Abstract]
Land	1,493
Buildings and Improvements	6,906
Total	8,399
Accumulated Depreciation at December 31, 2013	3,114
Depreciable Life (Years)	40 years
309 Fellowship Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,518
Buildings and Improvements	6,154
Net Improvements (Retirements) Since Acquisition	780
Carrying Amount at Which Carried [Abstract]
Land	1,518
Buildings and Improvements	6,934
Total	8,452
Accumulated Depreciation at December 31, 2013	3,070
Depreciable Life (Years)	40 years
1000 Bishops Gate [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	934
Buildings and Improvements	6,287
Net Improvements (Retirements) Since Acquisition	(647)
Carrying Amount at Which Carried [Abstract]
Land	934
Buildings and Improvements	5,640
Total	6,574
Accumulated Depreciation at December 31, 2013	1,534
Depreciable Life (Years)	40 years
161 Gaither Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,016
Buildings and Improvements	4,064
Net Improvements (Retirements) Since Acquisition	731
Carrying Amount at Which Carried [Abstract]
Land	1,016
Buildings and Improvements	4,795
Total	5,811
Accumulated Depreciation at December 31, 2013	1,855
Depreciable Life (Years)	40 years
815 East Gate Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	636
Buildings and Improvements	2,584
Net Improvements (Retirements) Since Acquisition	550
Carrying Amount at Which Carried [Abstract]
Land	636
Buildings and Improvements	3,134
Total	3,770
Accumulated Depreciation at December 31, 2013	1,436
Depreciable Life (Years)	40 years
4000 Midlantic Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	714
Buildings and Improvements	5,085
Net Improvements (Retirements) Since Acquisition	(1,833)
Carrying Amount at Which Carried [Abstract]
Land	714
Buildings and Improvements	3,252
Total	3,966
Accumulated Depreciation at December 31, 2013	1,531
Depreciable Life (Years)	40 years
817 East Gate Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	611
Buildings and Improvements	2,426
Net Improvements (Retirements) Since Acquisition	263
Carrying Amount at Which Carried [Abstract]
Land	611
Buildings and Improvements	2,689
Total	3,300
Accumulated Depreciation at December 31, 2013	1,159
Depreciable Life (Years)	40 years
400 Commerce Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,528
Buildings and Improvements	9,220
Net Improvements (Retirements) Since Acquisition	1,086
Carrying Amount at Which Carried [Abstract]
Land	2,528
Buildings and Improvements	10,306
Total	12,834
Accumulated Depreciation at December 31, 2013	3,911
Depreciable Life (Years)	40 years
100 Commerce Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,160
Buildings and Improvements	4,633
Net Improvements (Retirements) Since Acquisition	859
Carrying Amount at Which Carried [Abstract]
Land	1,160
Buildings and Improvements	5,492
Total	6,652
Accumulated Depreciation at December 31, 2013	2,595
Depreciable Life (Years)	40 years
200 Commerce Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	911
Buildings and Improvements	4,414
Net Improvements (Retirements) Since Acquisition	1,018
Carrying Amount at Which Carried [Abstract]
Land	911
Buildings and Improvements	5,432
Total	6,343
Accumulated Depreciation at December 31, 2013	2,180
Depreciable Life (Years)	40 years
Main Street-Plaza 1000 [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,732
Buildings and Improvements	10,942
Net Improvements (Retirements) Since Acquisition	2,933
Carrying Amount at Which Carried [Abstract]
Land	2,732
Buildings and Improvements	13,875
Total	16,607
Accumulated Depreciation at December 31, 2013	13,211
Depreciable Life (Years)	40 years
Main Street-Piazza [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	696
Buildings and Improvements	2,802
Net Improvements (Retirements) Since Acquisition	3,034
Carrying Amount at Which Carried [Abstract]
Land	696
Buildings and Improvements	5,836
Total	6,532
Accumulated Depreciation at December 31, 2013	1,714
Depreciable Life (Years)	40 years
Main Street-Promenade [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	531
Buildings and Improvements	2,052
Net Improvements (Retirements) Since Acquisition	444
Carrying Amount at Which Carried [Abstract]
Land	531
Buildings and Improvements	2,496
Total	3,027
Accumulated Depreciation at December 31, 2013	1,218
Depreciable Life (Years)	40 years
920 North King Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,141
Buildings and Improvements	21,140
Net Improvements (Retirements) Since Acquisition	2,669
Carrying Amount at Which Carried [Abstract]
Land	6,141
Buildings and Improvements	23,809
Total	29,950
Accumulated Depreciation at December 31, 2013	7,669
Depreciable Life (Years)	30 years
300 Delaware Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,368
Buildings and Improvements	13,739
Net Improvements (Retirements) Since Acquisition	2,548
Carrying Amount at Which Carried [Abstract]
Land	6,368
Buildings and Improvements	16,287
Total	22,655
Accumulated Depreciation at December 31, 2013	6,642
Depreciable Life (Years)	23 years
Two Righter Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,802
Buildings and Improvements	11,217
Net Improvements (Retirements) Since Acquisition	6,172
Carrying Amount at Which Carried [Abstract]
Land	2,802
Buildings and Improvements	17,389
Total	20,191
Accumulated Depreciation at December 31, 2013	4,822
Depreciable Life (Years)	40 years
One Righter Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,545
Buildings and Improvements	10,195
Net Improvements (Retirements) Since Acquisition	5,864
Carrying Amount at Which Carried [Abstract]
Land	2,545
Buildings and Improvements	16,059
Total	18,604
Accumulated Depreciation at December 31, 2013	8,839
Depreciable Life (Years)	40 years
4364 South Alston Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,622
Buildings and Improvements	6,419
Net Improvements (Retirements) Since Acquisition	1,357
Carrying Amount at Which Carried [Abstract]
Land	1,581
Buildings and Improvements	7,817
Total	9,398
Accumulated Depreciation at December 31, 2013	3,174
Depreciable Life (Years)	40 years
4805 Lake Brooke Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,640
Buildings and Improvements	6,567
Net Improvements (Retirements) Since Acquisition	1,537
Carrying Amount at Which Carried [Abstract]
Land	1,640
Buildings and Improvements	8,104
Total	9,744
Accumulated Depreciation at December 31, 2013	3,243
Depreciable Life (Years)	40 years
Overlook I [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	748
Buildings and Improvements	3,976
Net Improvements (Retirements) Since Acquisition	599
Carrying Amount at Which Carried [Abstract]
Land	790
Buildings and Improvements	4,533
Total	5,323
Accumulated Depreciation at December 31, 2013	677
Depreciable Life (Years)	40 years
Overlook II [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	748
Buildings and Improvements	4,492
Net Improvements (Retirements) Since Acquisition	364
Carrying Amount at Which Carried [Abstract]
Land	790
Buildings and Improvements	4,814
Total	5,604
Accumulated Depreciation at December 31, 2013	671
Depreciable Life (Years)	40 years
2812 Emerywood Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,069
Buildings and Improvements	4,281
Net Improvements (Retirements) Since Acquisition	515
Carrying Amount at Which Carried [Abstract]
Land	1,069
Buildings and Improvements	4,796
Total	5,865
Accumulated Depreciation at December 31, 2013	2,182
Depreciable Life (Years)	40 years
300 Arboretum Place [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	5,450
Buildings and Improvements	21,892
Net Improvements (Retirements) Since Acquisition	6,917
Carrying Amount at Which Carried [Abstract]
Land	5,450
Buildings and Improvements	28,809
Total	34,259
Accumulated Depreciation at December 31, 2013	10,862
Depreciable Life (Years)	40 years
7501 Boulders View Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,669
Buildings and Improvements	19,699
Net Improvements (Retirements) Since Acquisition	2,071
Carrying Amount at Which Carried [Abstract]
Land	4,925
Buildings and Improvements	21,514
Total	26,439
Accumulated Depreciation at December 31, 2013	4,786
Depreciable Life (Years)	40 years
7300 Beaufont Springs Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,672
Buildings and Improvements	19,689
Net Improvements (Retirements) Since Acquisition	2,857
Carrying Amount at Which Carried [Abstract]
Land	4,922
Buildings and Improvements	22,296
Total	27,218
Accumulated Depreciation at December 31, 2013	4,223
Depreciable Life (Years)	40 years
6800 Paragon Place [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,552
Buildings and Improvements	18,414
Net Improvements (Retirements) Since Acquisition	2,163
Carrying Amount at Which Carried [Abstract]
Land	4,552
Buildings and Improvements	20,577
Total	25,129
Accumulated Depreciation at December 31, 2013	4,651
Depreciable Life (Years)	40 years
6802 Paragon Place [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,917
Buildings and Improvements	11,454
Net Improvements (Retirements) Since Acquisition	3,143
Carrying Amount at Which Carried [Abstract]
Land	2,917
Buildings and Improvements	14,597
Total	17,514
Accumulated Depreciation at December 31, 2013	5,312
Depreciable Life (Years)	40 years
1025 Boulders Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,574
Buildings and Improvements	11,297
Net Improvements (Retirements) Since Acquisition	1,704
Carrying Amount at Which Carried [Abstract]
Land	2,824
Buildings and Improvements	12,751
Total	15,575
Accumulated Depreciation at December 31, 2013	2,545
Depreciable Life (Years)	40 years
2100-2116 West Laburnam Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,482
Buildings and Improvements	8,846
Net Improvements (Retirements) Since Acquisition	2,770
Carrying Amount at Which Carried [Abstract]
Land	2,482
Buildings and Improvements	11,616
Total	14,098
Accumulated Depreciation at December 31, 2013	5,162
Depreciable Life (Years)	40 years
7401 Beaufont Springs Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,349
Buildings and Improvements	10,396
Net Improvements (Retirements) Since Acquisition	1,011
Carrying Amount at Which Carried [Abstract]
Land	2,599
Buildings and Improvements	11,157
Total	13,756
Accumulated Depreciation at December 31, 2013	2,289
Depreciable Life (Years)	40 years
7325 Beaufont Springs Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,344
Buildings and Improvements	10,377
Net Improvements (Retirements) Since Acquisition	630
Carrying Amount at Which Carried [Abstract]
Land	2,594
Buildings and Improvements	10,757
Total	13,351
Accumulated Depreciation at December 31, 2013	2,049
Depreciable Life (Years)	40 years
9011 Arboretum Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,857
Buildings and Improvements	7,702
Net Improvements (Retirements) Since Acquisition	940
Carrying Amount at Which Carried [Abstract]
Land	1,857
Buildings and Improvements	8,642
Total	10,499
Accumulated Depreciation at December 31, 2013	3,854
Depreciable Life (Years)	40 years
6806 Paragon Place [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	10,288
Net Improvements (Retirements) Since Acquisition	(37)
Carrying Amount at Which Carried [Abstract]
Land	402
Buildings and Improvements	9,849
Total	10,251
Accumulated Depreciation at December 31, 2013	2,660
Depreciable Life (Years)	40 years
9100 Arboretum Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,362
Buildings and Improvements	5,489
Net Improvements (Retirements) Since Acquisition	979
Carrying Amount at Which Carried [Abstract]
Land	1,362
Buildings and Improvements	6,468
Total	7,830
Accumulated Depreciation at December 31, 2013	2,807
Depreciable Life (Years)	40 years
2511 Brittons Hill Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,202
Buildings and Improvements	4,820
Net Improvements (Retirements) Since Acquisition	1,221
Carrying Amount at Which Carried [Abstract]
Land	1,202
Buildings and Improvements	6,041
Total	7,243
Accumulated Depreciation at December 31, 2013	2,846
Depreciable Life (Years)	40 years
100 Gateway Centre Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	391
Buildings and Improvements	5,410
Net Improvements (Retirements) Since Acquisition	617
Carrying Amount at Which Carried [Abstract]
Land	391
Buildings and Improvements	6,027
Total	6,418
Accumulated Depreciation at December 31, 2013	1,922
Depreciable Life (Years)	40 years
9200 Arboretum Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	985
Buildings and Improvements	3,973
Net Improvements (Retirements) Since Acquisition	1,252
Carrying Amount at Which Carried [Abstract]
Land	985
Buildings and Improvements	5,225
Total	6,210
Accumulated Depreciation at December 31, 2013	2,348
Depreciable Life (Years)	40 years
9210 Arboretum Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,110
Buildings and Improvements	4,474
Net Improvements (Retirements) Since Acquisition	517
Carrying Amount at Which Carried [Abstract]
Land	1,110
Buildings and Improvements	4,991
Total	6,101
Accumulated Depreciation at December 31, 2013	2,239
Depreciable Life (Years)	40 years
2201-2245 Tomlynn Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,020
Buildings and Improvements	4,067
Net Improvements (Retirements) Since Acquisition	1,046
Carrying Amount at Which Carried [Abstract]
Land	1,020
Buildings and Improvements	5,113
Total	6,133
Accumulated Depreciation at December 31, 2013	2,154
Depreciable Life (Years)	40 years
9211 Arboretum Parkway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	582
Buildings and Improvements	2,433
Net Improvements (Retirements) Since Acquisition	844
Carrying Amount at Which Carried [Abstract]
Land	582
Buildings and Improvements	3,277
Total	3,859
Accumulated Depreciation at December 31, 2013	1,317
Depreciable Life (Years)	40 years
2221-2245 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	530
Buildings and Improvements	2,123
Net Improvements (Retirements) Since Acquisition	359
Carrying Amount at Which Carried [Abstract]
Land	530
Buildings and Improvements	2,482
Total	3,012
Accumulated Depreciation at December 31, 2013	990
Depreciable Life (Years)	40 years
2212-2224 Tomlynn Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	502
Buildings and Improvements	2,014
Net Improvements (Retirements) Since Acquisition	395
Carrying Amount at Which Carried [Abstract]
Land	502
Buildings and Improvements	2,409
Total	2,911
Accumulated Depreciation at December 31, 2013	1,020
Depreciable Life (Years)	40 years
2244 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	550
Buildings and Improvements	2,203
Net Improvements (Retirements) Since Acquisition	0
Carrying Amount at Which Carried [Abstract]
Land	550
Buildings and Improvements	2,203
Total	2,753
Accumulated Depreciation at December 31, 2013	964
Depreciable Life (Years)	40 years
2277 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	507
Buildings and Improvements	2,034
Net Improvements (Retirements) Since Acquisition	294
Carrying Amount at Which Carried [Abstract]
Land	507
Buildings and Improvements	2,328
Total	2,835
Accumulated Depreciation at December 31, 2013	977
Depreciable Life (Years)	40 years
2248 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	512
Buildings and Improvements	2,049
Net Improvements (Retirements) Since Acquisition	57
Carrying Amount at Which Carried [Abstract]
Land	512
Buildings and Improvements	2,106
Total	2,618
Accumulated Depreciation at December 31, 2013	923
Depreciable Life (Years)	40 years
2161-2179 Tomlynn Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	423
Buildings and Improvements	1,695
Net Improvements (Retirements) Since Acquisition	315
Carrying Amount at Which Carried [Abstract]
Land	423
Buildings and Improvements	2,010
Total	2,433
Accumulated Depreciation at December 31, 2013	842
Depreciable Life (Years)	40 years
2246 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	455
Buildings and Improvements	1,822
Net Improvements (Retirements) Since Acquisition	0
Carrying Amount at Which Carried [Abstract]
Land	455
Buildings and Improvements	1,822
Total	2,277
Accumulated Depreciation at December 31, 2013	797
Depreciable Life (Years)	40 years
2251 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	387
Buildings and Improvements	1,552
Net Improvements (Retirements) Since Acquisition	95
Carrying Amount at Which Carried [Abstract]
Land	387
Buildings and Improvements	1,647
Total	2,034
Accumulated Depreciation at December 31, 2013	738
Depreciable Life (Years)	40 years
2256 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	356
Buildings and Improvements	1,427
Net Improvements (Retirements) Since Acquisition	182
Carrying Amount at Which Carried [Abstract]
Land	356
Buildings and Improvements	1,609
Total	1,965
Accumulated Depreciation at December 31, 2013	751
Depreciable Life (Years)	40 years
2130-2146 Tomlynn Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	353
Buildings and Improvements	1,416
Net Improvements (Retirements) Since Acquisition	214
Carrying Amount at Which Carried [Abstract]
Land	353
Buildings and Improvements	1,630
Total	1,983
Accumulated Depreciation at December 31, 2013	704
Depreciable Life (Years)	40 years
2120 Tomlynn Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	281
Buildings and Improvements	1,125
Net Improvements (Retirements) Since Acquisition	300
Carrying Amount at Which Carried [Abstract]
Land	281
Buildings and Improvements	1,425
Total	1,706
Accumulated Depreciation at December 31, 2013	625
Depreciable Life (Years)	40 years
2240 Dabney Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	264
Buildings and Improvements	1,059
Net Improvements (Retirements) Since Acquisition	88
Carrying Amount at Which Carried [Abstract]
Land	264
Buildings and Improvements	1,147
Total	1,411
Accumulated Depreciation at December 31, 2013	478
Depreciable Life (Years)	40 years
Boulders Land [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	1,256
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	28
Carrying Amount at Which Carried [Abstract]
Land	1,259
Buildings and Improvements	25
Total	1,284
Accumulated Depreciation at December 31, 2013	2
5900 & 5950 La Place Court [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,706
Buildings and Improvements	11,185
Net Improvements (Retirements) Since Acquisition	3,613
Carrying Amount at Which Carried [Abstract]
Land	3,955
Buildings and Improvements	14,549
Total	18,504
Accumulated Depreciation at December 31, 2013	3,645
Depreciable Life (Years)	48 years
5963 La Place Court [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	2,824
Buildings and Improvements	9,413
Net Improvements (Retirements) Since Acquisition	2,117
Carrying Amount at Which Carried [Abstract]
Land	2,999
Buildings and Improvements	11,355
Total	14,354
Accumulated Depreciation at December 31, 2013	2,486
Depreciable Life (Years)	55 years
2035 Corte Del Nogal [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	3,261
Buildings and Improvements	6,077
Net Improvements (Retirements) Since Acquisition	1,167
Carrying Amount at Which Carried [Abstract]
Land	3,499
Buildings and Improvements	7,006
Total	10,505
Accumulated Depreciation at December 31, 2013	2,038
Depreciable Life (Years)	39 years
1200 Concord Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,395
Buildings and Improvements	24,664
Net Improvements (Retirements) Since Acquisition	808
Carrying Amount at Which Carried [Abstract]
Land	6,515
Buildings and Improvements	25,352
Total	31,867
Accumulated Depreciation at December 31, 2013	7,949
Depreciable Life (Years)	34 years
1220 Concord Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	6,476
Buildings and Improvements	24,966
Net Improvements (Retirements) Since Acquisition	260
Carrying Amount at Which Carried [Abstract]
Land	6,476
Buildings and Improvements	25,226
Total	31,702
Accumulated Depreciation at December 31, 2013	7,622
Depreciable Life (Years)	34 years
155 Grand Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	13,556
Buildings and Improvements	54,267
Net Improvements (Retirements) Since Acquisition	6,316
Carrying Amount at Which Carried [Abstract]
Land	13,556
Buildings and Improvements	60,583
Total	74,139
Accumulated Depreciation at December 31, 2013	12,652
Depreciable Life (Years)	40 years
Two Kaiser Plaza [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	7,841
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	0
Carrying Amount at Which Carried [Abstract]
Land	7,841
Buildings and Improvements	0
Total	7,841
Accumulated Depreciation at December 31, 2013	0
Oakland Lot B [Member]
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances at December 31, 2012	0
Initial Cost [Abstract]
Land	4,342
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	0
Carrying Amount at Which Carried [Abstract]
Land	4,342
Buildings and Improvements	0
Total	4,342
Accumulated Depreciation at December 31, 2013	$ 0