Real Estate Investments - Dispositions (Textual) (Details)	3 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) property	Dec. 31, 2016 USD ($) a property Parcel	Dec. 31, 2015 USD ($) a property Parcel	Dec. 31, 2014 USD ($) ft² a property Parcel
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property			113
Impairment of properties held for use			$ 7,300,000
Aggregate carrying value			25,800,000
Estimated Fair Value Of Impaired Property			$ 18,500,000
1000 Atrium Way [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²					99,668
Libertyview [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | 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
Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Parcels | Parcel			3	4	2
Acreage of land | a			122.9	14.7	22.1
Other Segment [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			$ 7,300,000
Carrying value of real estate prior to impairment			25,800,000
Aggregate carrying value			$ 18,500,000
Impairment Hold Period			10 years
Residual Capitalization Rates			8.75%
Discount Rates			9.00%
Other Segment [Member] | Land [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment of properties held for use			$ 5,600,000
Carrying value of real estate prior to impairment			18,200,000
Aggregate carrying value			$ 12,600,000
Number of Parcels | Parcel			5
Acreage of land | a			108
Metropolitan DC [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties | property		2		3
Impairment of properties held for use	$ 3,900,000	$ 7,400,000		$ 27,500,000
Carrying value of real estate prior to impairment	37,400,000	51,900,000
Aggregate carrying value	$ 33,500,000	$ 44,500,000		$ 40,400,000
Impairment Hold Period	10 years	10 years		10 years
Residual Capitalization Rates	7.75%	7.00%		8.00%
Discount Rates	8.25%	7.00%		8.00%
Estimated Fair Value Of Impaired Property				$ 12,900,000