Debt - Long-Term Debt (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Mar. 17, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Unsecured revolving credit facility, net	$ 546,778,000		$ 0
Deferred financing costs, net	(13,007,000)		(11,452,000)
Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	608,871,000		610,821,000
Total principal	2,523,871,000		1,675,821,000
Deferred financing costs, net	(13,007,000)		(7,247,000)
Total	2,510,864,000		1,668,574,000
Metro Center | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 88,526,000		89,650,000
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)	4.75%
1010 Third Avenue and 77 West 55th Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 37,868,000		38,251,000
Stated Rate (as a percent)	4.01%
Effective Rate (as a percent)	4.20%
10 Bank Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 32,477,000		32,920,000
Stated Rate (as a percent)	4.23%
Effective Rate (as a percent)	4.36%
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)	4.29%
Senior unsecured notes, Series A | Senior 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 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 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 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 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 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 notes
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 H | Senior notes
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
Unsecured revolving credit facility | Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility, net	$ 550,000,000		0
Effective Rate (as a percent)	2.17%
Unsecured revolving credit facility | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.10%
Unsecured term loan facility | Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility, net	$ 215,000,000		265,000,000
Effective Rate (as a percent)	3.61%
Unsecured term loan facility | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.20%
Unsecured term loan facility | Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility, net	$ 175,000,000		$ 0
Effective Rate (as a percent)	2.98%
Unsecured term loan facility | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.50%
First Stamford Place - First Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Stated Rate (as a percent)	4.09%
Face amount	$ 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	$ 16,000,000