Real Estate Investments - Summary of Office Properties Sold (Details) $ in Thousands			3 Months Ended
		Feb. 04, 2016 property	Mar. 31, 2016 USD ($) ft² property		Mar. 31, 2015 USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net Proceeds on Sale			$ 704,911		$ 26,778
Gain (Loss) on Sale			$ 115,456		$ 9,019
Office Building [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property			59
Rentable Square Feet | ft²			4,787,475
Sales Price			$ 752,100
Net Proceeds on Sale			704,121
Gain (Loss) on Sale	[1]		$ 115,456
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
Sales Price			$ 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	[2]
Rentable Square Feet | ft²	[2]		3,924,783
Sales Price	[2]		$ 398,100
Net Proceeds on Sale	[2]		353,971
Gain (Loss) on Sale	[1],[2],[3]		$ (372)

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

[2] Exhibit 99.2 to Form 8-K filed on February 10, 2016 contains a complete list of the 58 properties. See also Note 4, "Investment in Unconsolidated Real Estate Ventures," for further details of the transactions.

[3] During the fourth quarter of 2015, the Company recognized an impairment loss totaling approximately $45.4 million on the properties. The loss on sale represents additional closing costs that were not known at settlement.