Properties Held for Sale and Property Dispositions (Details) $ in Thousands	1 Months Ended																	12 Months Ended
	Nov. 30, 2018 USD ($) ft² a	Jun. 30, 2018 USD ($) ft²	May 31, 2018 USD ($) ft²	Feb. 22, 2018	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 30, 2017	Oct. 31, 2017 USD ($) ft²	Sep. 30, 2017 USD ($)	Jul. 31, 2017 USD ($) ft²	Apr. 30, 2017 USD ($) ft²	Oct. 31, 2016 USD ($) ft²	Aug. 31, 2016 USD ($) ft²	Jul. 31, 2016 USD ($) ft²	Jun. 30, 2016 USD ($) ft²	May 31, 2016 USD ($) a	Feb. 29, 2016 USD ($) ft²	Dec. 31, 2018 USD ($) ft²	Feb. 28, 2018 USD ($)	Jan. 01, 2018 USD ($) ft²	Dec. 31, 2016 USD ($)
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²																		30,177,253
Employee-related liabilities						$ 0												$ 11,700			$ 1,100
2 Herald Square
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	369,000
Gross asset valuation	$ 265,000
Gain on Sale	$ 0
Ownership percentage in disposed asset	49.00%
400 Summit Lake Drive
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | a	39.5
Gross asset valuation	$ 3,000
Gain on Sale	$ (36,200)
Upper East Side Assemblage
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²		70,142
Gross asset valuation		$ 143,800
Gain on Sale		$ (6,300)
Impairment charge																		$ 5,800
1-6 International Drive
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²		540,000
Gross asset valuation		$ 55,000
Gain on Sale		$ (2,600)
635 Madison Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²		176,530
Gross asset valuation		$ 153,000
Gain on Sale		$ (14,100)
115-117 Stevens Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²			178,000
Gross asset valuation			$ 12,000
Gain on Sale			$ (700)
600 Lexington Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²																				303,515
Gross asset valuation																				$ 305,000
Gain on Sale					$ 23,800
Employee-related liabilities					$ 1,300
1515 Broadway
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²																		1,750,000
Gross asset valuation																		$ 1,950,000	$ 1,950,000
Gain on Sale						$ 0
Joint venture ownership percentage sold				12.87%			30.13%
125 Chubb Way
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²								278,000
Gross asset valuation								$ 29,500
Gain on Sale								$ (26,100)
16 Court Street
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²								317,600
Gross asset valuation								$ 171,000
Gain on Sale								64,900
Employee-related liabilities								$ 2,500
680-750 Washington Boulevard
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²										325,000
Gross asset valuation										$ 97,000
Gain on Sale										$ (44,200)
520 White Plains Road
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²											180,000
Gross asset valuation											$ 21,000
Gain on Sale											$ (14,600)
102 Greene Street
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²											9,200
Gross asset valuation									$ 43,500		$ 43,500
Gain on Sale											4,900
Employee-related liabilities											$ 900
Joint venture ownership percentage sold									10.00%		90.00%
400 East 57th Street
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²												290,482
Gross asset valuation												$ 83,300
Gain on Sale												23,900
Employee-related liabilities												$ 1,000
Ownership percentage in disposed asset												49.00%
11 Madison Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²													2,314,000
Gross asset valuation													$ 2,605,000
Gain on Sale													3,600
Employee-related liabilities													$ 600
Ownership percentage in disposed asset													40.00%
500 West Putnam Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²														121,500
Gross asset valuation														$ 41,000
Gain on Sale														$ (10,400)
388 Greenwich
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²															2,635,000
Gross asset valuation															$ 2,002,300
Gain on Sale															206,500
Employee-related liabilities															$ 1,600
7 International Drive
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | a																31
Gross asset valuation																$ 20,000
Gain on Sale																$ (6,900)
248-252 Bedford Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²																	66,611
Gross asset valuation																	$ 55,000
Gain on Sale																	15,300
Employee-related liabilities																	$ 1,300
885 Third Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²																	607,000
Gross asset valuation																	$ 453,000
Gain on Sale																	$ (8,800)