Properties Held for Sale and Property Dispositions (Details) $ in Thousands	1 Months Ended																12 Months Ended
	Dec. 31, 2019 USD ($) ft²	Nov. 30, 2019 USD ($) ft²	Aug. 31, 2019 USD ($) ft²	Nov. 30, 2018 USD ($) ft² a	Oct. 31, 2018 USD ($) ft²	Jul. 31, 2018 USD ($) ft²	Jun. 30, 2018 USD ($) ft²	May 31, 2018 USD ($) ft²	Feb. 28, 2018	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) ft²	Nov. 30, 2017	Oct. 31, 2017 USD ($) ft²	Sep. 30, 2017	Jul. 31, 2017 USD ($) ft²	Apr. 30, 2017 USD ($) ft²	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) ft²	Sep. 30, 2019 USD ($)	Jan. 01, 2018 USD ($) ft²
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	27,607,971																27,607,971
Employee-related liabilities	$ 4,000										$ 0						$ 4,000	$ 11,700	$ 0
Purchase price and other fair value adjustments																	$ 69,389	$ 57,385	$ 0
East 220 Street 42
Property Dispositions and Assets Held for Sale
Gross asset valuation																				$ 815,000
Suburban
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	1,107,000																1,107,000
Gross asset valuation	$ 229,200																$ 229,200
Gain on Sale	$ 1,800
1640 Flatbush Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	1,000																1,000
Gross asset valuation	$ 16,200																$ 16,200
Gain on Sale	5,500
Employee-related liabilities	$ 2,000																$ 2,000
562 Fifth Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	42,635																42,635
Gross asset valuation	$ 52,400																$ 52,400
Gain on Sale	$ (26,600)
1010 Washington Boulevard
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²		143,400
Gross asset valuation		$ 23,100
Gain on Sale		$ (7,100)
Spring Street 115
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²			5,218
Gross asset valuation			$ 66,600
Gain on Sale			3,600
Purchase price and other fair value adjustments			$ 3,800
Ownership percentage in disposed asset			49.00%														49.00%
Deconsolidation of ownership interest percentage			51.00%														51.00%
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								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
Gain on Sale																4,900
Employee-related liabilities																$ 900
Ownership percentage in disposed asset																90.00%
Joint venture ownership percentage sold														10.00%
102 Greene Street
Property Dispositions and Assets Held for Sale
Investment in joint venture, ownership percentage																10.00%
Development Rights | Upper East Side Assemblage
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²					31,076