SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 76,634
Initial Cost to Company
Land	286,511
Buildings and Improvements	2,133,579
Costs Capitalized Subsequent to Acquisition, Net	504,021
Impairment/Write Downs	(67,221)
Cost Amount Carried at Close of Period
Land	286,186
Buildings and Improvements	2,570,704
Total	2,856,890
Accumulated Depreciation	755,255	$ 898,939	$ 1,030,445	$ 895,059
Parkshore Plaza, Folsom, CA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	41,275
Initial Cost to Company
Land	4,370
Buildings and Improvements	41,749
Costs Capitalized Subsequent to Acquisition, Net	5,630
Impairment/Write Downs	(6,310)
Cost Amount Carried at Close of Period
Land	3,841
Buildings and Improvements	41,598
Total	45,439
Accumulated Depreciation	6,695
1225 Seventeenth Street, Denver, CO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	22,400
Buildings and Improvements	110,090
Costs Capitalized Subsequent to Acquisition, Net	24,798
Impairment/Write Downs	(485)
Cost Amount Carried at Close of Period
Land	22,400
Buildings and Improvements	134,403
Total	156,803
Accumulated Depreciation	25,990
5073, 5075, & 5085 S. Syracuse Street, Denver, CO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	4,720
Buildings and Improvements	58,890
Costs Capitalized Subsequent to Acquisition, Net	0
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	4,720
Buildings and Improvements	58,890
Total	63,610
Accumulated Depreciation	9,815
1601 Dry Creek Drive, Longmont, CO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,714
Buildings and Improvements	24,397
Costs Capitalized Subsequent to Acquisition, Net	6,439
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	3,715
Buildings and Improvements	30,835
Total	34,550
Accumulated Depreciation	10,012
97 Newberry Road, East Windsor, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	5,903
Initial Cost to Company
Land	2,960
Buildings and Improvements	12,360
Costs Capitalized Subsequent to Acquisition, Net	30
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	2,943
Buildings and Improvements	12,407
Total	15,350
Accumulated Depreciation	3,175
33 Stiles Lane, North Haven, CT
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	2,415
Initial Cost to Company
Land	2,090
Buildings and Improvements	9,141
Costs Capitalized Subsequent to Acquisition, Net	216
Impairment/Write Downs	(1,654)
Cost Amount Carried at Close of Period
Land	1,799
Buildings and Improvements	7,994
Total	9,793
Accumulated Depreciation	2,353
1250 H Street, NW, Washington, DC
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	5,975
Buildings and Improvements	53,778
Costs Capitalized Subsequent to Acquisition, Net	13,103
Impairment/Write Downs	(248)
Cost Amount Carried at Close of Period
Land	5,975
Buildings and Improvements	66,633
Total	72,608
Accumulated Depreciation	28,384
Georgetown-Green and Harris Buildings, Washington, DC
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	24,000
Buildings and Improvements	35,979
Costs Capitalized Subsequent to Acquisition, Net	44
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	24,000
Buildings and Improvements	36,023
Total	60,023
Accumulated Depreciation	6,600
802 Delaware Avenue, Wilmington, DE
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	4,409
Buildings and Improvements	39,681
Costs Capitalized Subsequent to Acquisition, Net	10,562
Impairment/Write Downs	(11,156)
Cost Amount Carried at Close of Period
Land	3,390
Buildings and Improvements	40,106
Total	43,496
Accumulated Depreciation	24,217
6600 North Military Trail, Boca Raton, FL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	15,900
Buildings and Improvements	129,790
Costs Capitalized Subsequent to Acquisition, Net	118
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	15,900
Buildings and Improvements	129,908
Total	145,808
Accumulated Depreciation	19,471
625 Crane Street, Aurora, IL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	1,180
Buildings and Improvements	3,411
Costs Capitalized Subsequent to Acquisition, Net	(2)
Impairment/Write Downs	(4,589)
Cost Amount Carried at Close of Period
Land	0
Buildings and Improvements	0
Total	0
Accumulated Depreciation	0
600 West Chicago Avenue, Chicago, IL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	34,980
Buildings and Improvements	315,643
Costs Capitalized Subsequent to Acquisition, Net	44,821
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	34,980
Buildings and Improvements	360,464
Total	395,444
Accumulated Depreciation	45,839
8750 Bryn Mawr Avenue, Chicago, IL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	6,600
Buildings and Improvements	77,764
Costs Capitalized Subsequent to Acquisition, Net	9,498
Impairment/Write Downs	(257)
Cost Amount Carried at Close of Period
Land	6,600
Buildings and Improvements	87,005
Total	93,605
Accumulated Depreciation	14,411
109 Brookline Avenue, Boston, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,168
Buildings and Improvements	30,397
Costs Capitalized Subsequent to Acquisition, Net	14,090
Impairment/Write Downs	(119)
Cost Amount Carried at Close of Period
Land	3,168
Buildings and Improvements	44,368
Total	47,536
Accumulated Depreciation	20,196
Cabot Business Park Land, Mansfield, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	1,033
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition, Net	0
Impairment/Write Downs	(458)
Cost Amount Carried at Close of Period
Land	575
Buildings and Improvements	0
Total	575
Accumulated Depreciation	0
111 Market Place, Baltimore, MD
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	6,328
Buildings and Improvements	54,645
Costs Capitalized Subsequent to Acquisition, Net	17,781
Impairment/Write Downs	(7,199)
Cost Amount Carried at Close of Period
Land	5,814
Buildings and Improvements	65,741
Total	71,555
Accumulated Depreciation	27,356
25 S. Charles Street, Baltimore, MD
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	2,830
Buildings and Improvements	22,996
Costs Capitalized Subsequent to Acquisition, Net	12,678
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	2,830
Buildings and Improvements	35,674
Total	38,504
Accumulated Depreciation	13,640
820 W. Diamond, Gaithersburg, MD
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	4,381
Buildings and Improvements	18,798
Costs Capitalized Subsequent to Acquisition, Net	10,516
Impairment/Write Downs	(35)
Cost Amount Carried at Close of Period
Land	4,461
Buildings and Improvements	29,199
Total	33,660
Accumulated Depreciation	12,777
Danac Stiles Business Park, Rockville, MD
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	7,638
Buildings and Improvements	62,572
Costs Capitalized Subsequent to Acquisition, Net	6,483
Impairment/Write Downs	(10,975)
Cost Amount Carried at Close of Period
Land	6,595
Buildings and Improvements	59,123
Total	65,718
Accumulated Depreciation	20,919
East Eisenhower Parkway, Ann Arbor, MI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	6,760
Buildings and Improvements	46,988
Costs Capitalized Subsequent to Acquisition, Net	2,628
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	6,760
Buildings and Improvements	49,616
Total	56,376
Accumulated Depreciation	8,189
2250 Pilot Knob Road, Mendota Heights, MN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	533
Buildings and Improvements	4,795
Costs Capitalized Subsequent to Acquisition, Net	1,202
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	533
Buildings and Improvements	5,997
Total	6,530
Accumulated Depreciation	2,896
411 Farwell Avenue, South St. Paul, MN
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	1,303
Buildings and Improvements	10,451
Costs Capitalized Subsequent to Acquisition, Net	4,603
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	1,304
Buildings and Improvements	15,053
Total	16,357
Accumulated Depreciation	4,056
4700 Belleview Avenue, Kansas City, MO
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	1,165
Buildings and Improvements	3,097
Costs Capitalized Subsequent to Acquisition, Net	2,989
Impairment/Write Downs	(26)
Cost Amount Carried at Close of Period
Land	1,165
Buildings and Improvements	6,060
Total	7,225
Accumulated Depreciation	1,286
Cherrington Corporate Center Moon, Township, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	11,369
Buildings and Improvements	39,892
Costs Capitalized Subsequent to Acquisition, Net	28,319
Impairment/Write Downs	(7,616)
Cost Amount Carried at Close of Period
Land	10,664
Buildings and Improvements	61,300
Total	71,964
Accumulated Depreciation	22,494
1500 Market Street, Philadelphia, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	18,758
Buildings and Improvements	167,487
Costs Capitalized Subsequent to Acquisition, Net	119,060
Impairment/Write Downs	(759)
Cost Amount Carried at Close of Period
Land	18,758
Buildings and Improvements	285,788
Total	304,546
Accumulated Depreciation	88,717
1600 Market Street, Philadelphia, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,462
Buildings and Improvements	111,946
Costs Capitalized Subsequent to Acquisition, Net	20,637
Impairment/Write Downs	(194)
Cost Amount Carried at Close of Period
Land	3,462
Buildings and Improvements	132,389
Total	135,851
Accumulated Depreciation	59,435
1735 Market Street, Philadelphia, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	24,753
Buildings and Improvements	222,775
Costs Capitalized Subsequent to Acquisition, Net	67,076
Impairment/Write Downs	(12,071)
Cost Amount Carried at Close of Period
Land	24,747
Buildings and Improvements	277,786
Total	302,533
Accumulated Depreciation	118,405
Foster Plaza, Pittsburgh, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	6,168
Buildings and Improvements	51,588
Costs Capitalized Subsequent to Acquisition, Net	20,229
Impairment/Write Downs	(1,729)
Cost Amount Carried at Close of Period
Land	6,170
Buildings and Improvements	70,086
Total	76,256
Accumulated Depreciation	21,553
206 East 9th Street, Austin, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	27,041
Initial Cost to Company
Land	7,900
Buildings and Improvements	38,533
Costs Capitalized Subsequent to Acquisition, Net	2,776
Impairment/Write Downs	(12)
Cost Amount Carried at Close of Period
Land	7,900
Buildings and Improvements	41,297
Total	49,197
Accumulated Depreciation	4,879
4515 Seton Center Parkway, Austin, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	2,038
Buildings and Improvements	18,338
Costs Capitalized Subsequent to Acquisition, Net	2,814
Impairment/Write Downs	(60)
Cost Amount Carried at Close of Period
Land	2,037
Buildings and Improvements	21,093
Total	23,130
Accumulated Depreciation	9,749
4516 Seton Center Parkway, Austin, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	2,028
Buildings and Improvements	18,251
Costs Capitalized Subsequent to Acquisition, Net	4,003
Impairment/Write Downs	(25)
Cost Amount Carried at Close of Period
Land	2,027
Buildings and Improvements	22,230
Total	24,257
Accumulated Depreciation	10,497
Bridgepoint Square, Austin, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	7,784
Buildings and Improvements	70,526
Costs Capitalized Subsequent to Acquisition, Net	13,727
Impairment/Write Downs	(705)
Cost Amount Carried at Close of Period
Land	7,785
Buildings and Improvements	83,547
Total	91,332
Accumulated Depreciation	39,708
Research Park, Austin, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	15,859
Buildings and Improvements	60,175
Costs Capitalized Subsequent to Acquisition, Net	17,410
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	21,213
Buildings and Improvements	72,231
Total	93,444
Accumulated Depreciation	32,295
333 108th Avenue NE, Bellevue, WA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	14,400
Buildings and Improvements	136,412
Costs Capitalized Subsequent to Acquisition, Net	2,438
Impairment/Write Downs	(31)
Cost Amount Carried at Close of Period
Land	14,400
Buildings and Improvements	138,819
Total	153,219
Accumulated Depreciation	25,788
600 108th Avenue NE, Bellevue, WA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,555
Buildings and Improvements	30,244
Costs Capitalized Subsequent to Acquisition, Net	17,305
Impairment/Write Downs	(508)
Cost Amount Carried at Close of Period
Land	3,555
Buildings and Improvements	47,041
Total	50,596
Accumulated Depreciation	$ 13,458