SCHEDULE III REAL ESTATE AND ACCUMULATED DEPRECIATION (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 31,940
Initial Cost to Company
Land	186,443
Buildings and Improvements	1,304,951
Costs Capitalized Subsequent to Acquisition, Net	276,758
Impairment/Write Downs	(20,541)
Cost Amount Carried at Close of Period
Land	191,775
Buildings and Improvements	1,555,836
Total	1,747,611
Accumulated Depreciation	450,718	$ 755,255	$ 898,939	$ 1,030,445
1225 Seventeenth Street
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	28,273
Impairment/Write Downs	(1,563)
Cost Amount Carried at Close of Period
Land	22,400
Buildings and Improvements	136,800
Total	159,200
Accumulated Depreciation	31,093
5073, 5075, & 5085 S. Syracuse Street
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	11,287
1601 Dry Creek Drive
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	7,498
Impairment/Write Downs	(156)
Cost Amount Carried at Close of Period
Land	3,714
Buildings and Improvements	31,739
Total	35,453
Accumulated Depreciation	10,751
97 Newberry Road
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	5,404
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	3,485
1250 H Street, NW
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	14,906
Impairment/Write Downs	(248)
Cost Amount Carried at Close of Period
Land	5,975
Buildings and Improvements	68,436
Total	74,411
Accumulated Depreciation	30,990
Georgetown-Green and Harris Buildings
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	7,501
600 West Chicago Avenue
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	50,761
Impairment/Write Downs	(322)
Cost Amount Carried at Close of Period
Land	34,980
Buildings and Improvements	366,082
Total	401,062
Accumulated Depreciation	57,951
8750 Bryn Mawr Avenue
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	14,234
Impairment/Write Downs	(258)
Cost Amount Carried at Close of Period
Land	6,600
Buildings and Improvements	91,740
Total	98,340
Accumulated Depreciation	17,342
109 Brookline Avenue
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	15,233
Impairment/Write Downs	(1,397)
Cost Amount Carried at Close of Period
Land	3,168
Buildings and Improvements	44,233
Total	47,401
Accumulated Depreciation	20,494
777 East Eisenhower Parkway
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land	3,675
Buildings and Improvements	26,988
Costs Capitalized Subsequent to Acquisition, Net	4,485
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	3,675
Buildings and Improvements	31,473
Total	35,148
Accumulated Depreciation	5,821
1735 Market Street
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	81,484
Impairment/Write Downs	(14,927)
Cost Amount Carried at Close of Period
Land	24,748
Buildings and Improvements	289,337
Total	314,085
Accumulated Depreciation	125,376
206 East 9th Street
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	26,536
Initial Cost to Company
Land	7,900
Buildings and Improvements	38,533
Costs Capitalized Subsequent to Acquisition, Net	4,058
Impairment/Write Downs	(12)
Cost Amount Carried at Close of Period
Land	7,900
Buildings and Improvements	42,579
Total	50,479
Accumulated Depreciation	6,169
Bridgepoint Square
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	17,125
Impairment/Write Downs	(770)
Cost Amount Carried at Close of Period
Land	7,784
Buildings and Improvements	86,881
Total	94,665
Accumulated Depreciation	42,573
Research Park
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	17,439
Impairment/Write Downs	0
Cost Amount Carried at Close of Period
Land	21,213
Buildings and Improvements	72,260
Total	93,473
Accumulated Depreciation	34,941
333 108th Avenue NE
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	2,781
Impairment/Write Downs	(31)
Cost Amount Carried at Close of Period
Land	14,400
Buildings and Improvements	139,162
Total	153,562
Accumulated Depreciation	29,467
600 108th Avenue NE
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	18,407
Impairment/Write Downs	(857)
Cost Amount Carried at Close of Period
Land	3,555
Buildings and Improvements	47,794
Total	51,349
Accumulated Depreciation	$ 15,477