Real Estate Investments - Summary of Office Properties Sold (Details) $ in Thousands			3 Months Ended		9 Months Ended
		Feb. 04, 2016 property	Sep. 30, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) ft² property		Sep. 30, 2015 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net Proceeds on Sale					$ 758,931		$ 152,243
Gain (Loss) on Sale			$ (104)	$ 6,083	$ 114,625		$ 16,673
Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property					63
Rentable Square Feet | ft²			5,082,963		5,082,963
Sales Price			$ 807,200		$ 807,200
Net Proceeds on Sale					757,785
Gain (Loss) on Sale	[1]				$ 114,601
Office Building [Member] | 1120 Executive Plaza [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			95,183		95,183
Sales Price			$ 9,500		$ 9,500
Net Proceeds on Sale					9,241
Gain (Loss) on Sale	[1],[2]				$ (18)
Office Building [Member] | 50 East Clementon Road [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			3,080		3,080
Sales Price			$ 1,100		$ 1,100
Net Proceeds on Sale					1,011
Gain (Loss) on Sale	[1]				$ (85)
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],[3]				$ (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	[4]
Rentable Square Feet | ft²	[4]		3,924,783		3,924,783
Sales Price	[4]		$ 398,100		$ 398,100
Net Proceeds on Sale	[4]				353,971
Gain (Loss) on Sale	[1],[4],[5]				$ (372)

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

[2] As of June 30, 2016, the Company determined that the sale of the property was probable and classified this property as held for sale in accordance with applicable accounting standards for long lived assets. At such date, the carrying value of the property exceeded the fair value less the anticipated costs of sale. As a result, the Company recognized a provision for impairment totaling approximately $1.8 million during the three-month period ended June 30, 2016. The fair value measurement was based on the pricing in the purchase and sale agreement. As the pricing in the purchase and sale agreement is unobservable, the Company determined that the inputs utilized to determine fair value for this property falls within Level 3 in accordance with the fair value hierarchy established by Accounting Standards Codification (ASC) Topic 820, "Fair Value Measurements and Disclosures.” The loss on sale represents additional closing costs recognized at settlement.

[3] During the three-month period ended March 31, 2016, the Company recognized a provision for impairment totaling approximately $7.4 million on the properties. See “Held for Use Impairment” section below. The loss on sale primarily relates to additional closing costs recognized at settlement.

[4] 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 Group LLC. See Note 4, "Investment in Unconsolidated Real Estate Ventures," for further details of the transactions.

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