Debt - Long-Term Debt (Details) - USD ($)	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Outstanding borrowings	$ 0	$ 0
Total principal	2,148,831,000
Deferred financing costs, net	(21,322,000)	(17,367,000)
Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	783,831,000	786,884,000
Mortgages, senior notes, and unsecured term loan facilities, not including unsecured revolving credit facility
Debt Instrument [Line Items]
Total principal	2,148,831,000	2,151,884,000
Deferred financing costs, net	(13,491,000)	(15,235,000)
Total	2,135,340,000	2,136,649,000
Metro Center | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 85,627,000	87,382,000
Stated Rate (as a percent)	3.59%
Effective Rate (as a percent)	3.66%
10 Union Square | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 50,000,000	50,000,000
Stated Rate (as a percent)	3.70%
Effective Rate (as a percent)	3.97%
1542 Third Avenue | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 30,000,000	30,000,000
Stated Rate (as a percent)	4.29%
Effective Rate (as a percent)	4.53%
First Stamford Place | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 180,000,000	180,000,000
Stated Rate (as a percent)	4.28%
Effective Rate (as a percent)	4.74%
First Stamford Place - First Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Stated Rate (as a percent)	4.09%
Face amount of debt instrument	$ 164,000,000
First Stamford Place - Second Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Stated Rate (as a percent)	6.25%
Face amount of debt instrument	$ 16,000,000
1010 Third Avenue and 77 West 55th Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 36,875,000	37,477,000
Stated Rate (as a percent)	4.01%
Effective Rate (as a percent)	4.22%
250 West 57th Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 180,000,000	180,000,000
Stated Rate (as a percent)	2.83%
Effective Rate (as a percent)	3.23%
10 Bank Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 31,329,000	32,025,000
Stated Rate (as a percent)	4.23%
Effective Rate (as a percent)	4.37%
383 Main Avenue | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 30,000,000	30,000,000
Stated Rate (as a percent)	4.44%
Effective Rate (as a percent)	4.55%
1333 Broadway | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 160,000,000	160,000,000
Stated Rate (as a percent)	4.21%
Effective Rate (as a percent)	4.28%
Senior unsecured notes, Series A | Senior unsecured notes
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 B | Senior unsecured notes
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 C | Senior unsecured notes
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 D | Senior unsecured notes
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 E | Senior unsecured notes
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 F | Senior unsecured notes
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 G | Senior unsecured notes
Debt Instrument [Line Items]
Total	$ 100,000,000	100,000,000
Stated Rate (as a percent)	3.61%
Effective Rate (as a percent)	4.89%
Senior unsecured notes, Series H | Senior unsecured notes
Debt Instrument [Line Items]
Total	$ 75,000,000	75,000,000
Stated Rate (as a percent)	3.73%
Effective Rate (as a percent)	5.00%
Unsecured term loan facilities | Revolving credit facility
Debt Instrument [Line Items]
Outstanding borrowings	$ 215,000,000	215,000,000
Effective Rate (as a percent)	3.57%
Unsecured term loan facilities | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.20%
Unsecured revolving credit facility | Revolving credit facility
Debt Instrument [Line Items]
Outstanding borrowings	$ 0	0
Effective Rate (as a percent)	0.00%
Unsecured revolving credit facility | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.30%
Unsecured term loan facility | Revolving credit facility
Debt Instrument [Line Items]
Outstanding borrowings	$ 175,000,000	$ 175,000,000
Effective Rate (as a percent)	3.67%
Unsecured term loan facility | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.50%