Real Estate Investments - Dispositions (Textual) (Details)		12 Months Ended
	Aug. 05, 2013 USD ($) a	Dec. 31, 2015 USD ($) ft² property	Dec. 31, 2014 USD ($) ft² property	Dec. 31, 2013 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property		179
Impairment of properties held for use		$ 27,500,000
Aggregate carrying value		40,400,000
Estimated Fair Value Of Impaired Property		$ 12,900,000
Net rentable space of property sold | ft²		4,300,000
Net gain (loss) on sale of undepreciated real estate		$ 3,019,000	$ 1,184,000	$ (137,000)
1000 Atrium Way [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net rentable space of property sold | ft²			99,668
Libertyview [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net rentable space of property sold | ft²			121,737
A 100 Atrium Way and Liberty View [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property			2
Provision for impairment on assets held for sale			$ 0
Dabney Land, East [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Acreage of Land | a	8
Sales price of properties sold	$ 500,000
Net gain (loss) on sale of undepreciated real estate	$ (100,000)
Metropolitan DC [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property		3
Impairment of properties held for use		$ 27,500,000
Aggregate carrying value		40,400,000
Estimated Fair Value Of Impaired Property		$ 12,900,000
Impairment Hold Period		10 years
Residual Capitalization Rates		8.00%
Discount Rates		8.00%