Debt - Long-Term Debt (Details) - USD ($)	6 Months Ended
	Jun. 30, 2017	Aug. 31, 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	(12,817,000)				(10,904,000)
Total	724,312,000				759,016,000
Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	725,136,000				752,840,000
Total principal	1,590,136,000				1,617,840,000
Unamortized premiums, net of unamortized discount	(1,232,000)				905,000
Deferred financing costs, net	(9,405,000)				(6,414,000)
Total	1,579,499,000				1,612,331,000
1333 Broadway | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 67,138,000				67,656,000
Stated Rate (as a percent)	6.32%
Effective Rate (as a percent)	3.74%
1400 Broadway - First Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 67,181,000				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	$ 9,281,000				9,389,000
Stated Rate (as a percent)	3.35%
Effective Rate (as a percent)	3.35%
111 West 33rd Street - First Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 74,662,000				75,261,000
Stated Rate (as a percent)	6.01%
Effective Rate (as a percent)	3.31%
111 West 33rd Street - Second Lien | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 9,440,000				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	$ 37,458,000				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	$ 94,976,000				95,985,000
Stated Rate (as a percent)	3.59%
Effective Rate (as a percent)	3.65%
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%
Effective Rate (as a percent)	4.81%
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%
Face amount	$ 164,000,000.0
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
1010 Third Avenue and 77 West 55th Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 40,000,000				26,502,000
Stated Rate (as a percent)	4.01%
Effective Rate (as a percent)	4.50%
Face amount	$ 40,000,000.0
10 Bank Street | Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 35,000,000				31,544,000
Stated Rate (as a percent)	4.23%
Effective Rate (as a percent)	4.50%
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%
Effective Rate (as a percent)	4.88%
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)	3.98%
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)	4.14%
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)	4.23%
Unsecured revolving credit facility | Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0				0
Effective Rate (as a percent)	2.37%
Unsecured revolving credit facility | Revolving credit facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.15%
Unsecured term loan facility | Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 265,000,000				$ 265,000,000
Effective Rate (as a percent)	2.82%
Unsecured term loan facility | Revolving credit facility | Forecast
Debt Instrument [Line Items]
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)	1.60%