Debt - Long-Term Debt (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Aug. 31, 2017	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017	Dec. 31, 2016	Aug. 31, 2014
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0					$ 0
Deferred financing costs, net	(17,329,000)					(10,904,000)
Total	720,830,000					759,016,000
Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	723,221,000					752,840,000
Total principal	1,588,221,000					1,617,840,000
Unamortized premiums, net of unamortized discount	(2,301,000)					905,000
Deferred financing costs, net	(8,630,000)					(6,414,000)
Total	1,577,290,000					1,612,331,000
1333 Broadway | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 66,872,000					67,656,000
Stated Rate (as a percent)	6.32%
Effective Rate (as a percent)	3.73%
1400 Broadway - First Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 66,909,000					67,714,000
Stated Rate (as a percent)	6.12%
Effective Rate (as a percent)	3.37%
1400 Broadway - Second Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 9,226,000					9,389,000
Stated Rate (as a percent)	3.35%
Effective Rate (as a percent)	3.36%
111 West 33rd Street - First Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 74,356,000					75,261,000
Stated Rate (as a percent)	6.01%
Effective Rate (as a percent)	3.32%
111 West 33rd Street - Second Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 9,405,000					9,509,000
Stated Rate (as a percent)	6.56%
Effective Rate (as a percent)	3.62%
1350 Broadway | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 37,302,000					37,764,000
Stated Rate (as a percent)	5.87%
Effective Rate (as a percent)	3.71%
Metro Center | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 94,464,000					95,985,000
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					17,795,000
Stated Rate (as a percent)	4.29%			4.23%	4.29%
Effective Rate (as a percent)	4.68%
Face amount					$ 30,000,000.0
First Stamford Place | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 180,000,000					235,067,000
Stated Rate (as a percent)	4.28%		4.28%
Effective Rate (as a percent)	4.67%
Face amount			$ 180,000,000.0
First Stamford Place - First Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Stated Rate (as a percent)	4.09%		4.09%
Face amount	$ 164,000,000		$ 164,000,000.0
First Stamford Place - Second Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Stated Rate (as a percent)	6.25%		6.25%
Face amount	$ 16,000,000		$ 16,000,000.0
1010 Third Avenue and 77 West 55th Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 39,885,000					26,502,000
Stated Rate (as a percent)	4.01%		4.01%
Effective Rate (as a percent)	4.45%
Face amount			$ 40,000,000.0
10 Bank Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 34,802,000					31,544,000
Stated Rate (as a percent)	4.23%
Effective Rate (as a percent)	4.54%
Face amount				$ 35,000,000.0
383 Main Avenue | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 30,000,000					28,654,000
Stated Rate (as a percent)	4.44%		4.44%
Effective Rate (as a percent)	4.77%
Face amount			$ 30,000,000.0
Senior unsecured notes - exchangeable | Convertible Debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 250,000,000					250,000,000
Stated Rate (as a percent)	2.625%						2.625%
Effective Rate (as a percent)	3.93%
Face amount							$ 250,000,000.0
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)	4.00%
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)	4.17%
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)	4.26%
Unsecured revolving credit facility | Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0					0
Effective Rate (as a percent)	2.33%
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	$ 265,000,000					$ 265,000,000
Stated Rate (as a percent)	3.35%
Fixed pay rate (as a percent)		2.1485%
Unsecured term loan facility | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.20%