SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 26,000
Initial Cost to Company
Land	129,794
Buildings and Improvements	788,909
Costs Capitalized Subsequent to Acquisition, Net	243,438
Impairment/Write Downs	(22,499)
Cost Amount Carried at Close of Period
Land	135,142
Buildings and Improvements	1,004,500
Total	1,139,642
Accumulated Depreciation	375,968	$ 450,718	$ 755,255	$ 898,939
1225 Seventeenth Street
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	33,108
Impairment/Write Downs	(2,750)
Cost Amount Carried at Close of Period
Land	22,400
Buildings and Improvements	140,448
Total	162,848
Accumulated Depreciation	35,066
1250 H Street, NW
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	15,895
Impairment/Write Downs	(579)
Cost Amount Carried at Close of Period
Land	5,975
Buildings and Improvements	69,094
Total	75,069
Accumulated Depreciation	33,383
Georgetown-Green and Harris Buildings
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	1,076
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	24,000
Buildings and Improvements	37,055
Total	61,055
Accumulated Depreciation	8,402
109 Brookline Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	15,636
Impairment/Write Downs	(1,397)
Cost Amount Carried at Close of Period
Land	3,168
Buildings and Improvements	44,636
Total	47,804
Accumulated Depreciation	21,853
1735 Market Street
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	95,918
Impairment/Write Downs	(14,927)
Cost Amount Carried at Close of Period
Land	24,748
Buildings and Improvements	303,771
Total	328,519
Accumulated Depreciation	136,425
206 East 9th Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	26,000
Initial Cost to Company
Land	7,900
Buildings and Improvements	38,533
Costs Capitalized Subsequent to Acquisition, Net	5,362
Impairment/Write Downs	(444)
Cost Amount Carried at Close of Period
Land	7,900
Buildings and Improvements	43,451
Total	51,351
Accumulated Depreciation	7,073
Bridgepoint Square
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	20,637
Impairment/Write Downs	(1,172)
Cost Amount Carried at Close of Period
Land	7,784
Buildings and Improvements	89,991
Total	97,775
Accumulated Depreciation	45,146
Research Park
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	33,383
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	21,212
Buildings and Improvements	88,205
Total	109,417
Accumulated Depreciation	38,117
333 108th Avenue NE
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	2,746
Impairment/Write Downs	(31)
Cost Amount Carried at Close of Period
Land	14,400
Buildings and Improvements	139,127
Total	153,527
Accumulated Depreciation	33,117
600 108th Avenue NE
SEC Schedule, 12-28, Real Estate Companies, Investment in 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	19,677
Impairment/Write Downs	(1,199)
Cost Amount Carried at Close of Period
Land	3,555
Buildings and Improvements	48,722
Total	52,277
Accumulated Depreciation	$ 17,386