Properties Held for Sale and Property Dispositions - Property Dispositions (Details) $ in Thousands	1 Months Ended										3 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($) ft²	Sep. 30, 2015 USD ($) ft²	Aug. 31, 2015 USD ($) ft²	Jan. 31, 2015 USD ($) ft²	Nov. 30, 2014 USD ($) ft²	Jul. 31, 2014 USD ($) ft²	May. 31, 2014 USD ($) ft²	Sep. 30, 2013 USD ($) ft²	Aug. 31, 2013 USD ($) ft²	Feb. 28, 2013 USD ($) ft²	Sep. 30, 2015 USD ($) ft²	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²	35,581,060											35,581,060
Gain on Sale												$ 14,122	$ 163,059	$ 14,900
Revenues
Rental revenue												236	51,090	94,558
Escalation and reimbursement revenues												(127)	4,646	14,856
Other income												0	23	554
Total revenues												109	55,759	109,968
Operating expenses												(631)	7,772	20,568
Real estate taxes												250	7,156	16,521
Ground rent												0	3,001	7,975
Transaction related costs												(49)	89	2
Depreciable real estate reserves												0	0	2,150
Interest expense, net of interest income												109	12,652	19,782
Amortization of deferred financing costs												3	433	840
Depreciation and amortization												0	5,581	16,443
Total expenses												(318)	36,684	84,281
Net income from discontinued operations												$ 427	$ 19,075	$ 25,687
140-150 Grand Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²	215,100											215,100
Sales Price	$ 32,000
Gain on Sale	$ (20,100)
Impairment Charge											$ 19,200	$ 19,200
570 & 574 Fifth Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²	24,327											24,327
Sales Price	$ 125,400
Gain on Sale	24,600
Employee compensation award	$ 4,000
120 West 45th Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²		440,000									440,000
Sales Price		$ 365,000
Gain on Sale		58,600
Employee compensation award		$ 2,000
131-137 Spring Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²			68,342
Sales Price			$ 277,800
Gain on Sale			101,100
Employee compensation award			$ 4,100
Ownership percentage in disposed asset			80.00%
180 Maiden Lane
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²				1,090,000
Sales Price				$ 470,000
Gain on Sale				17,000
Employee compensation award				$ 800
2 Herald Square
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²					354,400
Sales Price					$ 365,000
Gain on Sale					18,800
Employee compensation award					$ 2,500
985-987 Third Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²						13,678
Sales Price						$ 68,700
Gain on Sale						29,800
Employee compensation award						$ 1,300
673 First Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²							422,000
Sales Price							$ 145,000
Gain on Sale							117,600
Employee compensation award							$ 3,400
300 Main Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²								130,000
Sales Price								$ 13,500
Gain on Sale								$ (2,200)
333 West 34th Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²									345,400
Sales Price									$ 220,300
Gain on Sale									13,800
Employee compensation award									$ 3,000
44 West 55th Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²										8,557
Sales Price										$ 6,300
Gain on Sale										$ 1,100