Properties Held for Sale and Property Dispositions (Details) $ in Thousands	1 Months Ended													12 Months Ended
	Dec. 31, 2020 USD ($) ft²	Sep. 30, 2020 USD ($) ft²	May 31, 2020 USD ($) ft²	Mar. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2019 USD ($) 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 ($)	May 31, 2018 USD ($) ft²	Jan. 31, 2018 USD ($)	Dec. 31, 2020 USD ($) ft²	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($) ft²	Jun. 30, 2019 USD ($) ft²	Jan. 01, 2018 USD ($) ft²
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	29,500,737													29,500,737
Employee-related liabilities	$ 0				$ 4,000									$ 0	$ 4,000	$ 11,700
Purchase price and other fair value adjustments														$ 187,522	69,389	57,385
30 East 40th Street
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	69,446													69,446
Gross asset valuation	$ 5,200													$ 5,200
Gain on Sale	$ (1,600)
1055 Washington Boulevard
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	182,000													182,000
Gross asset valuation	$ 23,800													$ 23,800
Gain on Sale	$ (11,500)
Williamsburg Terrace
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	52,000													52,000
Gross asset valuation	$ 32,000													$ 32,000
Gain on Sale	$ 11,800
410 Tenth Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²	638,000													638,000
Gross asset valuation	$ 952,500													$ 952,500
Gain on Sale	$ 56,400
400 East 58th Street
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²		140,000
Gross asset valuation		$ 62,000
Gain on Sale		$ 8,300
609 Fifth Avenue
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²			21,437
Gross asset valuation			$ 168,000
Gain on Sale			$ 63,300
315 West 33rd Street - The Olivia
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²																	492,987
Gross asset valuation																	$ 446,500
Gain on Sale				$ 71,800
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
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 (4)
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)
Impairment charge															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%
Deconsolidation of ownership interest percentage							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
Properties Sold In 2020, 2019 and 2018
Property Dispositions and Assets Held for Sale
Employee-related liabilities	$ 10,500				$ 2,000									$ 10,500	$ 2,000	$ 1,300
Development Rights | Upper East Side Assemblage
Property Dispositions and Assets Held for Sale
Unaudited Approximate Usable Square Feet | ft²									31,076