Properties Held for Sale and Property Dispositions - Property Dispositions (Details) $ in Thousands	1 Months Ended													3 Months Ended			12 Months Ended
	Oct. 31, 2016 USD ($)	Aug. 31, 2016 USD ($) ft²	Jul. 31, 2016 USD ($) ft²	Jun. 30, 2016 USD ($) ft²	May 31, 2016 USD ($)	Feb. 29, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) 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 ($) ft²	Jun. 30, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) ft²	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²																	32,922,082
Gain on Sale																	$ 0	$ 14,122	$ 163,059
Employee compensation award																	2,300
Revenues
Rental revenue																	0	236	51,090
Escalation and reimbursement revenues																	0	(127)	4,646
Other income																	0	0	23
Total revenues																	0	109	55,759
Operating expenses																	0	(631)	7,772
Real estate taxes																	0	250	7,156
Ground rent																	0	0	3,001
Transaction related costs																	0	(49)	89
Interest expense, net of interest income																	0	109	12,652
Amortization of deferred financing costs																	0	3	433
Depreciation and amortization																	0	0	5,581
Total expenses																	0	(318)	36,684
Net income from discontinued operations																	$ 0	$ 427	$ 19,075
400 East 57th Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²		290,482
Sales Price		$ 83,300
Gain on Sale		$ 23,900
Employee compensation award	$ 1,000
Ownership percentage in disposed asset	49.00%
11 Madison Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²		2,314,000
Sales Price		$ 2,605,000
Gain on Sale		3,600
Employee compensation award		$ 600
Ownership percentage in disposed asset		40.00%
500 West Putnam Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²			121,500
Sales Price			$ 41,000
Gain on Sale			$ (10,400)
388 Greenwich [Member]
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²				2,635,000										2,635,000
Sales Price				$ 2,002,300
Gain on Sale				206,500
Employee compensation award				$ 1,600
248-252 Bedford Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²						66,611
Sales Price						$ 55,000
Gain on Sale						15,300
Employee compensation award						$ 1,300
885 Third Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²						607,000
Sales Price						$ 453,000
Gain on Sale						$ 0	$ (6,600)
Impairment Charge															$ 6,600
140-150 Grand Street
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²																	215,100
Sales Price							32,000
Gain on Sale							(20,100)
Impairment Charge														$ 10,400		$ 19,200	$ 19,200
570 & 574 Fifth Avenue
Properties Sold [Abstract]
Approximate Usable Square Feet (sqft) | ft²																	24,327
Sales Price							125,400
Gain on Sale							24,600
Employee compensation award							$ 4,000
7 International Drive
Properties Sold [Abstract]
Sales Price					$ 20,000
Gain on Sale					$ (6,900)
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