Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 315,437
Initial Cost [Abstract]
Land	443,949
Buildings and Improvements	2,886,455
Net Improvements (Retirements) Since Acquisition	676,055
Carrying Amount at Which Carried [Abstract]
Land	489,702
Buildings and Improvements	3,516,757
Total	4,006,459	$ 3,951,719	$ 3,830,824	$ 3,658,438
Accumulated Depreciation at December 31, 2019	973,318	885,407	$ 913,297	$ 885,392
Pennsylvania | 400 Berwyn Park
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		2,657
Buildings and Improvements		4,462
Net Improvements (Retirements) Since Acquisition		12,841
Carrying Amount at Which Carried [Abstract]
Land		2,657
Buildings and Improvements		17,303
Total		19,960
Accumulated Depreciation at December 31, 2019		7,583
Pennsylvania | 300 Berwyn Park
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		2,206
Buildings and Improvements		13,422
Net Improvements (Retirements) Since Acquisition		4,145
Carrying Amount at Which Carried [Abstract]
Land		2,206
Buildings and Improvements		17,567
Total		19,773
Accumulated Depreciation at December 31, 2019		10,516
Pennsylvania | 1050 Westlakes Drive
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		2,611
Buildings and Improvements		10,445
Net Improvements (Retirements) Since Acquisition		2,210
Carrying Amount at Which Carried [Abstract]
Land		2,611
Buildings and Improvements		12,655
Total		15,266
Accumulated Depreciation at December 31, 2019		6,238
Pennsylvania | 1200 Swedesford Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		2,595
Buildings and Improvements		11,809
Net Improvements (Retirements) Since Acquisition		490
Carrying Amount at Which Carried [Abstract]
Land		2,595
Buildings and Improvements		12,299
Total		14,894
Accumulated Depreciation at December 31, 2019		6,848
Pennsylvania | 200 Berwyn Park
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		1,533
Buildings and Improvements		9,460
Net Improvements (Retirements) Since Acquisition		1,574
Carrying Amount at Which Carried [Abstract]
Land		1,533
Buildings and Improvements		11,034
Total		12,567
Accumulated Depreciation at December 31, 2019		6,569
Pennsylvania | 1180 Swedesford Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		2,086
Buildings and Improvements		8,342
Net Improvements (Retirements) Since Acquisition		2,865
Carrying Amount at Which Carried [Abstract]
Land		2,086
Buildings and Improvements		11,207
Total		13,293
Accumulated Depreciation at December 31, 2019		5,802
Pennsylvania | 100 Berwyn Park
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		1,180
Buildings and Improvements		7,290
Net Improvements (Retirements) Since Acquisition		1,419
Carrying Amount at Which Carried [Abstract]
Land		1,180
Buildings and Improvements		8,709
Total		9,889
Accumulated Depreciation at December 31, 2019		5,252
Pennsylvania | 1160 Swedesford Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		1,781
Buildings and Improvements		7,124
Net Improvements (Retirements) Since Acquisition		6,810
Carrying Amount at Which Carried [Abstract]
Land		2,045
Buildings and Improvements		13,670
Total		15,715
Accumulated Depreciation at December 31, 2019		6,698
Pennsylvania | 1100 Cassett Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		1,695
Buildings and Improvements		6,779
Net Improvements (Retirements) Since Acquisition		1,730
Carrying Amount at Which Carried [Abstract]
Land		1,695
Buildings and Improvements		8,509
Total		10,204
Accumulated Depreciation at December 31, 2019		4,044
Pennsylvania | Six Tower Bridge (181 Washington Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		6,927
Buildings and Improvements		14,722
Net Improvements (Retirements) Since Acquisition		2,511
Carrying Amount at Which Carried [Abstract]
Land		6,237
Buildings and Improvements		17,923
Total		24,160
Accumulated Depreciation at December 31, 2019		3,803
Pennsylvania | 52 Swedesford Square
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		4,241
Buildings and Improvements		16,579
Net Improvements (Retirements) Since Acquisition		5,193
Carrying Amount at Which Carried [Abstract]
Land		4,241
Buildings and Improvements		21,772
Total		26,013
Accumulated Depreciation at December 31, 2019		11,511
Pennsylvania | 640 Freedom Business Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		1,015
Buildings and Improvements		20,098
Net Improvements (Retirements) Since Acquisition		4,832
Carrying Amount at Which Carried [Abstract]
Land		414
Buildings and Improvements		25,531
Total		25,945
Accumulated Depreciation at December 31, 2019		14,251
Pennsylvania | 620 Freedom Business Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		666
Buildings and Improvements		13,118
Net Improvements (Retirements) Since Acquisition		1,826
Carrying Amount at Which Carried [Abstract]
Land		270
Buildings and Improvements		15,340
Total		15,610
Accumulated Depreciation at December 31, 2019		8,836
Pennsylvania | 1000 First Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		0
Buildings and Improvements		13,708
Net Improvements (Retirements) Since Acquisition		3,211
Carrying Amount at Which Carried [Abstract]
Land		0
Buildings and Improvements		16,919
Total		16,919
Accumulated Depreciation at December 31, 2019		10,766
Pennsylvania | 1060 First Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		0
Buildings and Improvements		13,665
Net Improvements (Retirements) Since Acquisition		4,169
Carrying Amount at Which Carried [Abstract]
Land		0
Buildings and Improvements		17,834
Total		17,834
Accumulated Depreciation at December 31, 2019		11,055
Pennsylvania | 630 Freedom Business Center Drive
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		666
Buildings and Improvements		13,251
Net Improvements (Retirements) Since Acquisition		3,259
Carrying Amount at Which Carried [Abstract]
Land		273
Buildings and Improvements		16,903
Total		17,176
Accumulated Depreciation at December 31, 2019		9,861
Pennsylvania | 1020 First Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		0
Buildings and Improvements		10,744
Net Improvements (Retirements) Since Acquisition		3,931
Carrying Amount at Which Carried [Abstract]
Land		0
Buildings and Improvements		14,675
Total		14,675
Accumulated Depreciation at December 31, 2019		9,089
Pennsylvania | 1040 First Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		0
Buildings and Improvements		14,142
Net Improvements (Retirements) Since Acquisition		5,080
Carrying Amount at Which Carried [Abstract]
Land		0
Buildings and Improvements		19,222
Total		19,222
Accumulated Depreciation at December 31, 2019		12,076
Pennsylvania | 610 Freedom Business Center Drive
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		485
Buildings and Improvements		9,602
Net Improvements (Retirements) Since Acquisition		2,676
Carrying Amount at Which Carried [Abstract]
Land		198
Buildings and Improvements		12,565
Total		12,763
Accumulated Depreciation at December 31, 2019		6,976
Pennsylvania | 650 Park Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		1,916
Buildings and Improvements		4,378
Net Improvements (Retirements) Since Acquisition		(4,378)
Carrying Amount at Which Carried [Abstract]
Land		1,916
Buildings and Improvements		0
Total		1,916
Accumulated Depreciation at December 31, 2019		0
Pennsylvania | 600 Park Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		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, 2019		2,647
Pennsylvania | 933 First Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,127
Buildings and Improvements		20,794
Net Improvements (Retirements) Since Acquisition		(1,125)
Carrying Amount at Which Carried [Abstract]
Land		3,127
Buildings and Improvements		19,669
Total		22,796
Accumulated Depreciation at December 31, 2019		2,216
Pennsylvania | 500 North Gulph Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		1,303
Buildings and Improvements		5,201
Net Improvements (Retirements) Since Acquisition		21,017
Carrying Amount at Which Carried [Abstract]
Land		1,303
Buildings and Improvements		26,218
Total		27,521
Accumulated Depreciation at December 31, 2019		1,202
Pennsylvania | 401 Plymouth Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		6,199
Buildings and Improvements		16,131
Net Improvements (Retirements) Since Acquisition		15,831
Carrying Amount at Which Carried [Abstract]
Land		6,199
Buildings and Improvements		31,962
Total		38,161
Accumulated Depreciation at December 31, 2019		15,355
Pennsylvania | Metroplex (4000 Chemical Road)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		4,373
Buildings and Improvements		24,546
Net Improvements (Retirements) Since Acquisition		399
Carrying Amount at Which Carried [Abstract]
Land		4,373
Buildings and Improvements		24,945
Total		29,318
Accumulated Depreciation at December 31, 2019		7,701
Pennsylvania | 610 West Germantown Pike
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,651
Buildings and Improvements		14,514
Net Improvements (Retirements) Since Acquisition		3,285
Carrying Amount at Which Carried [Abstract]
Land		3,651
Buildings and Improvements		17,799
Total		21,450
Accumulated Depreciation at December 31, 2019		8,669
Pennsylvania | 600 West Germantown Pike
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,652
Buildings and Improvements		15,288
Net Improvements (Retirements) Since Acquisition		2,717
Carrying Amount at Which Carried [Abstract]
Land		3,652
Buildings and Improvements		18,005
Total		21,657
Accumulated Depreciation at December 31, 2019		8,124
Pennsylvania | 630 West Germantown Pike
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,558
Buildings and Improvements		14,743
Net Improvements (Retirements) Since Acquisition		2,714
Carrying Amount at Which Carried [Abstract]
Land		3,558
Buildings and Improvements		17,457
Total		21,015
Accumulated Depreciation at December 31, 2019		7,906
Pennsylvania | 620 West Germantown Pike
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,572
Buildings and Improvements		14,435
Net Improvements (Retirements) Since Acquisition		1,498
Carrying Amount at Which Carried [Abstract]
Land		3,572
Buildings and Improvements		15,933
Total		19,505
Accumulated Depreciation at December 31, 2019		7,042
Pennsylvania | 660 West Germantown Pike
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,694
Buildings and Improvements		5,487
Net Improvements (Retirements) Since Acquisition		20,715
Carrying Amount at Which Carried [Abstract]
Land		5,405
Buildings and Improvements		24,491
Total		29,896
Accumulated Depreciation at December 31, 2019		6,850
Pennsylvania | 351 Plymouth Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		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, 2019		205
Pennsylvania | 150 Radnor Chester Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		11,925
Buildings and Improvements		36,986
Net Improvements (Retirements) Since Acquisition		10,142
Carrying Amount at Which Carried [Abstract]
Land		11,897
Buildings and Improvements		47,156
Total		59,053
Accumulated Depreciation at December 31, 2019		22,504
Pennsylvania | One Radnor Corporate Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		7,323
Buildings and Improvements		28,613
Net Improvements (Retirements) Since Acquisition		22,885
Carrying Amount at Which Carried [Abstract]
Land		7,323
Buildings and Improvements		51,498
Total		58,821
Accumulated Depreciation at December 31, 2019		30,974
Pennsylvania | 201 King of Prussia Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		8,956
Buildings and Improvements		29,811
Net Improvements (Retirements) Since Acquisition		2,871
Carrying Amount at Which Carried [Abstract]
Land		8,949
Buildings and Improvements		32,689
Total		41,638
Accumulated Depreciation at December 31, 2019		19,061
Pennsylvania | 555 Lancaster Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		8,014
Buildings and Improvements		16,508
Net Improvements (Retirements) Since Acquisition		20,204
Carrying Amount at Which Carried [Abstract]
Land		8,609
Buildings and Improvements		36,117
Total		44,726
Accumulated Depreciation at December 31, 2019		18,535
Pennsylvania | Four Radnor Corporate Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		5,406
Buildings and Improvements		21,390
Net Improvements (Retirements) Since Acquisition		12,770
Carrying Amount at Which Carried [Abstract]
Land		5,705
Buildings and Improvements		33,861
Total		39,566
Accumulated Depreciation at December 31, 2019		15,781
Pennsylvania | Five Radnor Corporate Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		6,506
Buildings and Improvements		25,525
Net Improvements (Retirements) Since Acquisition		8,209
Carrying Amount at Which Carried [Abstract]
Land		6,578
Buildings and Improvements		33,662
Total		40,240
Accumulated Depreciation at December 31, 2019		12,120
Pennsylvania | Three Radnor Corporate Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		4,773
Buildings and Improvements		17,961
Net Improvements (Retirements) Since Acquisition		1,273
Carrying Amount at Which Carried [Abstract]
Land		4,791
Buildings and Improvements		19,216
Total		24,007
Accumulated Depreciation at December 31, 2019		9,598
Pennsylvania | Two Radnor Corporate Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,937
Buildings and Improvements		15,484
Net Improvements (Retirements) Since Acquisition		3,163
Carrying Amount at Which Carried [Abstract]
Land		3,942
Buildings and Improvements		18,642
Total		22,584
Accumulated Depreciation at December 31, 2019		9,853
Pennsylvania | 130 Radnor Chester Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		2,573
Buildings and Improvements		8,338
Net Improvements (Retirements) Since Acquisition		3,188
Carrying Amount at Which Carried [Abstract]
Land		2,567
Buildings and Improvements		11,532
Total		14,099
Accumulated Depreciation at December 31, 2019		6,716
Pennsylvania | 170 Radnor Chester Road
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		2,514
Buildings and Improvements		8,147
Net Improvements (Retirements) Since Acquisition		1,751
Carrying Amount at Which Carried [Abstract]
Land		2,509
Buildings and Improvements		9,903
Total		12,412
Accumulated Depreciation at December 31, 2019		4,375
Pennsylvania | 200 N. Radnor Chester Rd.
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,366
Buildings and Improvements		0
Net Improvements (Retirements) Since Acquisition		3,653
Carrying Amount at Which Carried [Abstract]
Land		3,366
Buildings and Improvements		3,653
Total		7,019
Accumulated Depreciation at December 31, 2019		772
Pennsylvania | 101 West Elm Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		6,251
Buildings and Improvements		25,209
Net Improvements (Retirements) Since Acquisition		3,853
Carrying Amount at Which Carried [Abstract]
Land		6,251
Buildings and Improvements		29,062
Total		35,313
Accumulated Depreciation at December 31, 2019		10,721
Pennsylvania | 1 West Elm Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		0
Initial Cost [Abstract]
Land		3,557
Buildings and Improvements		14,249
Net Improvements (Retirements) Since Acquisition		3,376
Carrying Amount at Which Carried [Abstract]
Land		3,557
Buildings and Improvements		17,625
Total		21,182
Accumulated Depreciation at December 31, 2019		6,823
Pennsylvania | Four Tower Bridge (200 Barr Harbor Drive)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances		9,291
Initial Cost [Abstract]
Land		6,000
Buildings and Improvements		14,734
Net Improvements (Retirements) Since Acquisition		336
Carrying Amount at Which Carried [Abstract]
Land		6,000
Buildings and Improvements		15,070
Total		21,070
Accumulated Depreciation at December 31, 2019		$ 761
Philadelphia CBD | Cira Centre (2929 Arch Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	208,570
Net Improvements (Retirements) Since Acquisition	(17,837)
Carrying Amount at Which Carried [Abstract]
Land	12,586
Buildings and Improvements	178,147
Total	190,733
Accumulated Depreciation at December 31, 2019	62,966
Philadelphia CBD | Three Logan Square (1717 Arch Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	98,188
Net Improvements (Retirements) Since Acquisition	75,116
Carrying Amount at Which Carried [Abstract]
Land	25,195
Buildings and Improvements	148,109
Total	173,304
Accumulated Depreciation at December 31, 2019	40,161
Philadelphia CBD | Two Commerce Square (2001 Market Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	108,472
Initial Cost [Abstract]
Land	15,323
Buildings and Improvements	120,200
Net Improvements (Retirements) Since Acquisition	33,693
Carrying Amount at Which Carried [Abstract]
Land	15,323
Buildings and Improvements	153,893
Total	169,216
Accumulated Depreciation at December 31, 2019	27,952
Philadelphia CBD | One Logan Square (130 North 18th Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	14,496
Buildings and Improvements	107,736
Net Improvements (Retirements) Since Acquisition	30,019
Carrying Amount at Which Carried [Abstract]
Land	14,473
Buildings and Improvements	137,778
Total	152,251
Accumulated Depreciation at December 31, 2019	62,425
Philadelphia CBD | Two Logan Square (100 North 18th Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	81,103
Initial Cost [Abstract]
Land	16,066
Buildings and Improvements	100,255
Net Improvements (Retirements) Since Acquisition	20,061
Carrying Amount at Which Carried [Abstract]
Land	16,066
Buildings and Improvements	120,316
Total	136,382
Accumulated Depreciation at December 31, 2019	49,266
Philadelphia CBD | One Commerce Square (2005 Market Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	116,571
Initial Cost [Abstract]
Land	15,161
Buildings and Improvements	105,021
Net Improvements (Retirements) Since Acquisition	32,326
Carrying Amount at Which Carried [Abstract]
Land	15,160
Buildings and Improvements	137,348
Total	152,508
Accumulated Depreciation at December 31, 2019	25,304
Philadelphia CBD | Cira Centre South Garage (129 South 30th Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	76,008
Net Improvements (Retirements) Since Acquisition	26,832
Carrying Amount at Which Carried [Abstract]
Land	6,905
Buildings and Improvements	95,935
Total	102,840
Accumulated Depreciation at December 31, 2019	19,682
Philadelphia CBD | 1900 Market Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	7,768
Buildings and Improvements	17,263
Net Improvements (Retirements) Since Acquisition	63,292
Carrying Amount at Which Carried [Abstract]
Land	7,768
Buildings and Improvements	80,555
Total	88,323
Accumulated Depreciation at December 31, 2019	15,492
Philadelphia CBD | 3020 Market Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	21,417
Net Improvements (Retirements) Since Acquisition	8,272
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	29,689
Total	29,689
Accumulated Depreciation at December 31, 2019	10,492
Philadelphia CBD | 618-634 Market Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	13,365
Buildings and Improvements	5,791
Net Improvements (Retirements) Since Acquisition	2,176
Carrying Amount at Which Carried [Abstract]
Land	13,365
Buildings and Improvements	7,967
Total	21,332
Accumulated Depreciation at December 31, 2019	5,975
Philadelphia CBD | FMC Tower at Cira Centre South (2929 Walnut Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	400,294
Net Improvements (Retirements) Since Acquisition	13,296
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	413,590
Total	413,590
Accumulated Depreciation at December 31, 2019	47,285
Philadelphia CBD | 2100 Market Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	18,827
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	6,433
Carrying Amount at Which Carried [Abstract]
Land	18,854
Buildings and Improvements	6,406
Total	25,260
Accumulated Depreciation at December 31, 2019	1,027
Philadelphia CBD | 3000 Market Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	18,924
Buildings and Improvements	13,080
Net Improvements (Retirements) Since Acquisition	1,039
Carrying Amount at Which Carried [Abstract]
Land	18,924
Buildings and Improvements	14,119
Total	33,043
Accumulated Depreciation at December 31, 2019	2,979
Philadelphia CBD | The Bulletin Building (3025 Market Street)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	24,377
Net Improvements (Retirements) Since Acquisition	13,353
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	37,730
Total	37,730
Accumulated Depreciation at December 31, 2019	2,086
Philadelphia CBD | 3001-3003 JFK Boulevard
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	111
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	111
Total	111
Accumulated Depreciation at December 31, 2019	10
Philadelphia CBD | 3025 JFK Boulevard
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	0
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	0
Total	0
Accumulated Depreciation at December 31, 2019	0
Metropolitan DC | 6600 Rockledge Drive
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	37,421
Net Improvements (Retirements) Since Acquisition	9,270
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	46,691
Total	46,691
Accumulated Depreciation at December 31, 2019	14,871
Metropolitan DC | 2340 Dulles Corner Boulevard
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	16,345
Buildings and Improvements	65,379
Net Improvements (Retirements) Since Acquisition	524
Carrying Amount at Which Carried [Abstract]
Land	16,129
Buildings and Improvements	66,119
Total	82,248
Accumulated Depreciation at December 31, 2019	22,150
Metropolitan DC | 1676 International Drive
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	18,437
Buildings and Improvements	97,538
Net Improvements (Retirements) Since Acquisition	(962)
Carrying Amount at Which Carried [Abstract]
Land	18,785
Buildings and Improvements	96,228
Total	115,013
Accumulated Depreciation at December 31, 2019	25,451
Metropolitan DC | 8260 Greensboro Drive
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	7,952
Buildings and Improvements	33,964
Net Improvements (Retirements) Since Acquisition	4,955
Carrying Amount at Which Carried [Abstract]
Land	8,102
Buildings and Improvements	38,769
Total	46,871
Accumulated Depreciation at December 31, 2019	11,152
Metropolitan DC | 2273 Research Boulevard
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	5,167
Buildings and Improvements	31,110
Net Improvements (Retirements) Since Acquisition	5,799
Carrying Amount at Which Carried [Abstract]
Land	5,237
Buildings and Improvements	36,839
Total	42,076
Accumulated Depreciation at December 31, 2019	11,928
Metropolitan DC | 2275 Research Boulevard
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	5,059
Buildings and Improvements	29,668
Net Improvements (Retirements) Since Acquisition	7,588
Carrying Amount at Which Carried [Abstract]
Land	5,154
Buildings and Improvements	37,161
Total	42,315
Accumulated Depreciation at December 31, 2019	13,308
Metropolitan DC | 2277 Research Boulevard
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	4,649
Buildings and Improvements	26,952
Net Improvements (Retirements) Since Acquisition	18,853
Carrying Amount at Which Carried [Abstract]
Land	4,733
Buildings and Improvements	45,721
Total	50,454
Accumulated Depreciation at December 31, 2019	17,093
Metropolitan DC | 8521 Leesburg Pike
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	4,316
Buildings and Improvements	30,885
Net Improvements (Retirements) Since Acquisition	7,145
Carrying Amount at Which Carried [Abstract]
Land	4,397
Buildings and Improvements	37,949
Total	42,346
Accumulated Depreciation at December 31, 2019	11,925
Austin, Texas | 11501 Burnet Road - Building 1
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	3,755
Buildings and Improvements	22,702
Net Improvements (Retirements) Since Acquisition	115
Carrying Amount at Which Carried [Abstract]
Land	3,755
Buildings and Improvements	22,817
Total	26,572
Accumulated Depreciation at December 31, 2019	3,193
Austin, Texas | 11501 Burnet Road - Building 2
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	2,732
Buildings and Improvements	16,305
Net Improvements (Retirements) Since Acquisition	1,551
Carrying Amount at Which Carried [Abstract]
Land	2,732
Buildings and Improvements	17,856
Total	20,588
Accumulated Depreciation at December 31, 2019	2,920
Austin, Texas | 11501 Burnet Road - Building 3
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	3,688
Buildings and Improvements	22,348
Net Improvements (Retirements) Since Acquisition	94
Carrying Amount at Which Carried [Abstract]
Land	3,688
Buildings and Improvements	22,442
Total	26,130
Accumulated Depreciation at December 31, 2019	3,161
Austin, Texas | 11501 Burnet Road - Building 4
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	2,614
Buildings and Improvements	15,740
Net Improvements (Retirements) Since Acquisition	75
Carrying Amount at Which Carried [Abstract]
Land	2,614
Buildings and Improvements	15,815
Total	18,429
Accumulated Depreciation at December 31, 2019	2,242
Austin, Texas | 11501 Burnet Road - Building 5
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	3,689
Buildings and Improvements	22,354
Net Improvements (Retirements) Since Acquisition	118
Carrying Amount at Which Carried [Abstract]
Land	3,689
Buildings and Improvements	22,472
Total	26,161
Accumulated Depreciation at December 31, 2019	3,207
Austin, Texas | 11501 Burnet Road - Building 6
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	2,676
Buildings and Improvements	15,972
Net Improvements (Retirements) Since Acquisition	13,973
Carrying Amount at Which Carried [Abstract]
Land	2,676
Buildings and Improvements	29,945
Total	32,621
Accumulated Depreciation at December 31, 2019	3,408
Austin, Texas | 11501 Burnet Road - Building 8
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	1,400
Buildings and Improvements	7,422
Net Improvements (Retirements) Since Acquisition	1,547
Carrying Amount at Which Carried [Abstract]
Land	1,400
Buildings and Improvements	8,969
Total	10,369
Accumulated Depreciation at December 31, 2019	1,172
Austin, Texas | 11501 Burnet Road - Parking Garage
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	0
Buildings and Improvements	19,826
Net Improvements (Retirements) Since Acquisition	63
Carrying Amount at Which Carried [Abstract]
Land	0
Buildings and Improvements	19,889
Total	19,889
Accumulated Depreciation at December 31, 2019	3,628
Austin, Texas | Four Points Centre 3, 11120 Four Points Drive
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	1,140
Buildings and Improvements	0
Net Improvements (Retirements) Since Acquisition	40,564
Carrying Amount at Which Carried [Abstract]
Land	1,140
Buildings and Improvements	40,564
Total	41,704
Accumulated Depreciation at December 31, 2019	1,511
Austin, Texas | One Barton Skyway (1501 South MoPac Expressway)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	10,496
Buildings and Improvements	47,670
Net Improvements (Retirements) Since Acquisition	113
Carrying Amount at Which Carried [Abstract]
Land	10,495
Buildings and Improvements	47,784
Total	58,279
Accumulated Depreciation at December 31, 2019	1,378
Austin, Texas | Two Barton Skyway (1601 South MoPac Expressway)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	10,849
Buildings and Improvements	53,868
Net Improvements (Retirements) Since Acquisition	2,828
Carrying Amount at Which Carried [Abstract]
Land	10,848
Buildings and Improvements	56,697
Total	67,545
Accumulated Depreciation at December 31, 2019	1,810
Austin, Texas | Three Barton Skyway (1221 South MoPac Expressway)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	10,374
Buildings and Improvements	47,624
Net Improvements (Retirements) Since Acquisition	184
Carrying Amount at Which Carried [Abstract]
Land	10,373
Buildings and Improvements	47,809
Total	58,182
Accumulated Depreciation at December 31, 2019	1,343
Austin, Texas | Four Barton Skyway (1301 South MoPac Expressway)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	13,301
Buildings and Improvements	57,041
Net Improvements (Retirements) Since Acquisition	(40)
Carrying Amount at Which Carried [Abstract]
Land	13,300
Buildings and Improvements	57,002
Total	70,302
Accumulated Depreciation at December 31, 2019	1,583
Austin, Texas | Four Points Centre (11305 Four Points Drive)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	7,800
Buildings and Improvements	43,581
Net Improvements (Retirements) Since Acquisition	3,463
Carrying Amount at Which Carried [Abstract]
Land	7,800
Buildings and Improvements	47,044
Total	54,844
Accumulated Depreciation at December 31, 2019	1,427
Austin, Texas | River Place - Building 1 (6500 River Place Boulevard)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	2,004
Buildings and Improvements	17,680
Net Improvements (Retirements) Since Acquisition	476
Carrying Amount at Which Carried [Abstract]
Land	2,004
Buildings and Improvements	18,156
Total	20,160
Accumulated Depreciation at December 31, 2019	547
Austin, Texas | River Place - Building 2 (6500 River Place Boulevard)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	3,137
Buildings and Improvements	29,254
Net Improvements (Retirements) Since Acquisition	191
Carrying Amount at Which Carried [Abstract]
Land	3,137
Buildings and Improvements	29,445
Total	32,582
Accumulated Depreciation at December 31, 2019	816
Austin, Texas | River Place - Building 3 (6500 River Place Boulevard)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	3,064
Buildings and Improvements	26,705
Net Improvements (Retirements) Since Acquisition	12
Carrying Amount at Which Carried [Abstract]
Land	3,064
Buildings and Improvements	26,717
Total	29,781
Accumulated Depreciation at December 31, 2019	735
Austin, Texas | River Place - Building 4 (6500 River Place Boulevard
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	2,273
Buildings and Improvements	18,617
Net Improvements (Retirements) Since Acquisition	1,001
Carrying Amount at Which Carried [Abstract]
Land	2,273
Buildings and Improvements	19,618
Total	21,891
Accumulated Depreciation at December 31, 2019	674
Austin, Texas | River Place - Building 5 (6500 River Place Boulevard)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	1,752
Buildings and Improvements	14,315
Net Improvements (Retirements) Since Acquisition	(24)
Carrying Amount at Which Carried [Abstract]
Land	1,752
Buildings and Improvements	14,291
Total	16,043
Accumulated Depreciation at December 31, 2019	394
Austin, Texas | River Place - Building 6 (6500 River Place Boulevard)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	1,598
Buildings and Improvements	12,945
Net Improvements (Retirements) Since Acquisition	(24)
Carrying Amount at Which Carried [Abstract]
Land	1,598
Buildings and Improvements	12,921
Total	14,519
Accumulated Depreciation at December 31, 2019	356
Austin, Texas | River Place - Building 7 (6500 River Place Boulevard)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	1,801
Buildings and Improvements	16,486
Net Improvements (Retirements) Since Acquisition	1,051
Carrying Amount at Which Carried [Abstract]
Land	1,801
Buildings and Improvements	17,537
Total	19,338
Accumulated Depreciation at December 31, 2019	586
Austin, Texas | Quarry Lake II (4516 Seton Center Parkway)
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	3,970
Buildings and Improvements	30,546
Net Improvements (Retirements) Since Acquisition	821
Carrying Amount at Which Carried [Abstract]
Land	3,867
Buildings and Improvements	31,470
Total	35,337
Accumulated Depreciation at December 31, 2019	876
Other | 10 Foster Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	244
Buildings and Improvements	971
Net Improvements (Retirements) Since Acquisition	69
Carrying Amount at Which Carried [Abstract]
Land	244
Buildings and Improvements	1,040
Total	1,284
Accumulated Depreciation at December 31, 2019	624
Other | 7 Foster Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	231
Buildings and Improvements	921
Net Improvements (Retirements) Since Acquisition	31
Carrying Amount at Which Carried [Abstract]
Land	231
Buildings and Improvements	952
Total	1,183
Accumulated Depreciation at December 31, 2019	549
Other | 2 Foster Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	185
Buildings and Improvements	730
Net Improvements (Retirements) Since Acquisition	11
Carrying Amount at Which Carried [Abstract]
Land	185
Buildings and Improvements	741
Total	926
Accumulated Depreciation at December 31, 2019	741
Other | 4 Foster Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	183
Buildings and Improvements	726
Net Improvements (Retirements) Since Acquisition	6
Carrying Amount at Which Carried [Abstract]
Land	183
Buildings and Improvements	732
Total	915
Accumulated Depreciation at December 31, 2019	732
Other | 1 Foster Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	93
Buildings and Improvements	364
Net Improvements (Retirements) Since Acquisition	8
Carrying Amount at Which Carried [Abstract]
Land	93
Buildings and Improvements	372
Total	465
Accumulated Depreciation at December 31, 2019	372
Other | 5 U.S. Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	21
Buildings and Improvements	81
Net Improvements (Retirements) Since Acquisition	2
Carrying Amount at Which Carried [Abstract]
Land	21
Buildings and Improvements	83
Total	104
Accumulated Depreciation at December 31, 2019	83
Other | 5 Foster Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	9
Buildings and Improvements	32
Net Improvements (Retirements) Since Acquisition	3
Carrying Amount at Which Carried [Abstract]
Land	9
Buildings and Improvements	35
Total	44
Accumulated Depreciation at December 31, 2019	35
Other | Main Street-Plaza 1000
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	2,732
Buildings and Improvements	10,942
Net Improvements (Retirements) Since Acquisition	296
Carrying Amount at Which Carried [Abstract]
Land	2,732
Buildings and Improvements	11,238
Total	13,970
Accumulated Depreciation at December 31, 2019	11,008
Other | Main Street-Piazza
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	696
Buildings and Improvements	2,802
Net Improvements (Retirements) Since Acquisition	3,700
Carrying Amount at Which Carried [Abstract]
Land	704
Buildings and Improvements	6,494
Total	7,198
Accumulated Depreciation at December 31, 2019	3,515
Other | Main Street-Promenade
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	532
Buildings and Improvements	2,052
Net Improvements (Retirements) Since Acquisition	312
Carrying Amount at Which Carried [Abstract]
Land	532
Buildings and Improvements	2,364
Total	2,896
Accumulated Depreciation at December 31, 2019	1,350
Other | 920 North King Street
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	6,141
Buildings and Improvements	21,140
Net Improvements (Retirements) Since Acquisition	7,969
Carrying Amount at Which Carried [Abstract]
Land	6,141
Buildings and Improvements	29,109
Total	35,250
Accumulated Depreciation at December 31, 2019	13,046
Other | 300 Delaware Avenue
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost [Abstract]
Land	6,369
Buildings and Improvements	13,739
Net Improvements (Retirements) Since Acquisition	2,638
Carrying Amount at Which Carried [Abstract]
Land	6,369
Buildings and Improvements	16,377
Total	22,746
Accumulated Depreciation at December 31, 2019	$ 9,031