Properties Held for Sale and Property Dispositions - Property Dispositions (Details) $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) ft²	Aug. 31, 2015 USD ($) ft²	Jan. 31, 2015 USD ($) ft²	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²	35,843,296			35,843,296		35,843,296
Gain on Sale				$ 0	$ 29,507	$ 12,983	$ 144,242
Revenues
Rental revenue				0	9,504	236	44,676
Escalation and reimbursement revenues				0	(194)	(127)	4,413
Other income				0	0	0	21
Total revenues				0	9,310	109	49,110
Operating expenses				0	773	(631)	7,101
Real estate taxes				0	787	250	6,618
Ground rent				0	0	0	3,001
Transaction related costs				0	115	(49)	155
Interest expense, net of interest income				0	2,874	109	10,983
Amortization of deferred financing costs				0	48	3	369
Depreciation and amortization				0	678	0	5,434
Total expenses				0	5,275	(318)	33,661
Net income from discontinued operations				$ 0	$ 4,035	$ 427	$ 15,449
180 Maiden Lane
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²			1,090,000
Sales Price			$ 470,000
Gain on Sale			$ 17,000
131-137 Spring Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²		68,342
Sales Price		$ 277,800
Gain on Sale		$ 101,100
Ownership percentage in disposed asset		80.00%
120 West 45th Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²	440,000			440,000		440,000
Sales Price	$ 365,000
Gain on Sale	$ 58,600