Debt - Schedule of Long term Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Nov. 30, 2020	Mar. 17, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0			$ 0
Total principal	2,151,884,000
Deferred financing costs, net	(17,367,000)			(11,452,000)
Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate debt	786,884,000			610,821,000
Fixed rate mortgage debt | Metro Center
Debt Instrument [Line Items]
Fixed rate debt	$ 87,382,000			89,650,000
Stated Rate (as a percent)	3.59%
Effective rate (as a percent)	3.68%
Fixed rate mortgage debt | 10 Union Square
Debt Instrument [Line Items]
Fixed rate debt	$ 50,000,000			50,000,000
Stated Rate (as a percent)	3.70%
Effective rate (as a percent)	3.97%
Fixed rate mortgage debt | 1542 Third Avenue
Debt Instrument [Line Items]
Fixed rate debt	$ 30,000,000			30,000,000
Stated Rate (as a percent)	4.29%
Effective rate (as a percent)	4.53%
Fixed rate mortgage debt | First Stamford Place
Debt Instrument [Line Items]
Fixed rate debt	$ 180,000,000			180,000,000
Stated Rate (as a percent)	4.28%
Effective rate (as a percent)	4.78%
Fixed rate mortgage debt | First Stamford Place - First Lien
Debt Instrument [Line Items]
Stated Rate (as a percent)	4.09%
Face amount	$ 164,000,000
Fixed rate mortgage debt | First Stamford Place - Second Lien
Debt Instrument [Line Items]
Stated Rate (as a percent)	6.25%
Face amount	$ 16,000,000
Fixed rate mortgage debt | 1010 Third Avenue and 77 West 55th Street
Debt Instrument [Line Items]
Fixed rate debt	$ 37,477,000			38,251,000
Stated Rate (as a percent)	4.01%
Effective rate (as a percent)	4.23%
Fixed rate mortgage debt | 250 West 57th Street
Debt Instrument [Line Items]
Fixed rate debt	$ 180,000,000	$ 180,000,000.0		0
Stated Rate (as a percent)	2.83%	2.83%
Effective rate (as a percent)	3.27%
Fixed rate mortgage debt | 10 Bank Street
Debt Instrument [Line Items]
Fixed rate debt	$ 32,025,000			32,920,000
Stated Rate (as a percent)	4.23%
Effective rate (as a percent)	4.36%
Fixed rate mortgage debt | 383 Main Avenue
Debt Instrument [Line Items]
Fixed rate debt	$ 30,000,000			30,000,000
Stated Rate (as a percent)	4.44%
Effective rate (as a percent)	4.55%
Fixed rate mortgage debt | 1333 Broadway
Debt Instrument [Line Items]
Fixed rate debt	$ 160,000,000			160,000,000
Stated Rate (as a percent)	4.21%
Effective rate (as a percent)	4.29%
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series A
Debt Instrument [Line Items]
Total	$ 100,000,000			100,000,000
Stated Rate (as a percent)	3.93%
Effective rate (as a percent)	3.96%
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series B
Debt Instrument [Line Items]
Total	$ 125,000,000			125,000,000
Stated Rate (as a percent)	4.09%
Effective rate (as a percent)	4.12%
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series C
Debt Instrument [Line Items]
Total	$ 125,000,000			125,000,000
Stated Rate (as a percent)	4.18%
Effective rate (as a percent)	4.21%
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series D
Debt Instrument [Line Items]
Total	$ 115,000,000			115,000,000
Stated Rate (as a percent)	4.08%
Effective rate (as a percent)	4.11%
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series E
Debt Instrument [Line Items]
Total	$ 160,000,000			160,000,000
Stated Rate (as a percent)	4.26%
Effective rate (as a percent)	4.27%
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series F
Debt Instrument [Line Items]
Total	$ 175,000,000			175,000,000
Stated Rate (as a percent)	4.44%
Effective rate (as a percent)	4.45%
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series G
Debt Instrument [Line Items]
Total	$ 100,000,000			0
Stated Rate (as a percent)	3.61%		3.61%
Effective rate (as a percent)	4.89%
Face amount			$ 100,000,000
Senior unsecured notes - Series A, B, C, D, E, F, G and H | Series H
Debt Instrument [Line Items]
Total	$ 75,000,000			0
Stated Rate (as a percent)	3.73%		3.73%
Effective rate (as a percent)	5.00%
Face amount			$ 75,000,000
Mortgages, senior notes, and unsecured term loan facilities, not including unsecured revolving credit facility
Debt Instrument [Line Items]
Total principal	$ 2,151,884,000			1,675,821,000
Deferred financing costs, net	(15,235,000)			(7,247,000)
Total	2,136,649,000			1,668,574,000
Revolving Credit Facility | Unsecured revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0			0
Effective rate (as a percent)	0.00%
Revolving Credit Facility | Unsecured revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.10%
Revolving Credit Facility | Unsecured term loan facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 215,000,000			265,000,000
Effective rate (as a percent)	3.84%
Revolving Credit Facility | Unsecured term loan facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.20%
Revolving Credit Facility | Unsecured term loan facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 175,000,000			$ 0
Effective rate (as a percent)	3.04%
Revolving Credit Facility | Unsecured term loan facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.50%