Real Estate Properties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) ft² building	Dec. 31, 2014 USD ($) property building	Dec. 31, 2013 USD ($) property building
Real estate properties
Number of Buildings | building	91
Rentable square feet of properties | ft²	10,700,000
Net book value	$ 1,440,253	$ 1,462,689
Balance Sheets:
Real estate properties, net	3,071
Rents receivable	1
Other assets	26
Assets of discontinued operations	12,468	13,165
Other liabilities	43
Liabilities of discontinued operations	54	150
Statement of Operations:
Loss on asset impairment			$ (10,142)
Income (loss) from discontinued operations	(525)	3,498	(889)
Discontinued operations
Balance Sheets:
Real estate properties, net	12,260	12,260
Rents receivable		782
Other assets	208	123
Assets of discontinued operations	12,468	13,165
Other liabilities	54	150
Liabilities of discontinued operations	54	150
Statement of Operations:
Rental income	114	1,673	4,580
Real estate taxes	(92)	(427)	(678)
Utility expenses	(161)	(226)	(539)
Other operating expenses	(272)	(459)	(966)
Depreciation and amortization			(1,025)
General and administrative	(114)	(181)	(287)
Loss on asset impairment			(10,142)
Increase in carrying value of asset held for sale		2,344
Net gain on sale of properties		774	8,168
Income (loss) from discontinued operations	$ (525)	$ 3,498	$ (889)
One building | Discontinued Operations, Held-for-sale | Office | Falls Church, VA
Real estate properties
Number of Buildings | building	1
Rentable square feet of properties | ft²	164,746
Net book value	$ 12,282
Two buildings | Discontinued operations
Real estate properties
Number of properties sold | property		2	2
Number of Buildings | building		2	2