Real Estate Investments - Summary of Office Properties Sold (Details) $ in Thousands			3 Months Ended		6 Months Ended
		Feb. 04, 2016 property	Jun. 30, 2016 USD ($) ft²	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) ft² property		Jun. 30, 2015 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net Proceeds on Sale					$ 748,395		$ 80,268
Gain (Loss) on Sale			$ (727)	$ 1,571	$ 114,729		$ 10,590
Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					61
Rentable Square Feet | ft²			4,984,700		4,984,700
Sales Price			$ 796,600		$ 796,600
Net Proceeds on Sale					747,533
Gain (Loss) on Sale	[1]				$ 114,704
Office Building [Member] | 196/198 Van Buren Street (Herndon Metro Plaza I&II) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					2
Rentable Square Feet | ft²			197,225		197,225
Sales Price			$ 44,500		$ 44,500
Net Proceeds on Sale					43,412
Gain (Loss) on Sale	[1],[2]				$ (752)
Office Building [Member] | 2970 Market Street (Cira Square) [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					1
Rentable Square Feet | ft²			862,692		862,692
Sales Price			$ 354,000		$ 354,000
Net Proceeds on Sale					350,150
Gain (Loss) on Sale	[1]				$ 115,828
Office Building [Member] | Och Ziff Portfolio [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property		58			58	[3]
Rentable Square Feet | ft²	[3]		3,924,783		3,924,783
Sales Price	[3]		$ 398,100		$ 398,100
Net Proceeds on Sale	[3]				353,971
Gain (Loss) on Sale	[1],[3],[4]				$ (372)

[1] Gain/(Loss) on Sale is net of closing and other transaction related costs.

[2] During the three-month period ended March 31, 2016, the Company recognized an impairment loss totaling approximately $7.4 million on the properties. The loss on sale represents additional closing costs recognized at settlement.

[3] Exhibit 99.2 to the Company’s Current Report on Form 8-K filed on February 10, 2016 contains a complete list of the 58 properties disposed of in the transactions with Och-Ziff Capital Management. See Note 4, "Investment in Unconsolidated Real Estate Ventures," for further details of the transactions.

[4] During the three-month period ended December 31, 2015, the Company recognized an impairment loss totaling approximately $45.4 million. The loss on sale represents additional closing costs recognized at settlement.