Debt - Schedule of Long term Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018	Jan. 31, 2018	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	(17,407,000)							(10,904,000)
Total	717,164,000							759,016,000
Mortgages [Member]
Debt Instrument [Line Items]
Fixed rate debt	721,224,000							752,840,000
Total principal	1,701,224,000							1,617,840,000
Unamortized (discount) premiums, net of unamortized premiums (discount)	(3,370,000)							905,000
Deferred financing costs, net	(9,133,000)							(6,414,000)
Total	1,688,721,000							1,612,331,000
Mortgages [Member] | 1333 Broadway [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 66,602,000							67,656,000
Stated Rate	6.32%
Effective rate	3.73%
Mortgages [Member] | 1400 Broadway - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 66,632,000							67,714,000
Stated Rate	6.12%
Effective rate	3.37%
Mortgages [Member] | 1400 Broadway - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 9,172,000							9,389,000
Stated Rate	3.35%
Effective rate	3.36%
Mortgages [Member] | 111 West 33rd Street - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 74,045,000							75,261,000
Stated Rate	6.01%
Effective rate	3.34%
Mortgages [Member] | 111 West 33rd Street - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 9,369,000							9,509,000
Stated Rate	6.56%
Effective rate	3.64%
Mortgages [Member] | 1350 Broadway [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 37,144,000							37,764,000
Stated Rate	5.87%
Effective rate	3.71%
Mortgages [Member] | Metro Center [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 93,948,000							95,985,000
Stated Rate	3.59%
Effective rate	3.67%
Mortgages [Member] | 10 Union Square [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 50,000,000							50,000,000
Stated Rate	3.70%
Effective rate	3.97%
Mortgages [Member] | 10 Bank Street [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 34,602,000							31,544,000
Stated Rate	4.23%					4.23%
Effective rate	4.49%
Face amount						$ 35,000,000.0
Mortgages [Member] | 1542 Third Avenue [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 30,000,000							17,795,000
Stated Rate	4.29%						4.29%
Effective rate	4.64%
Face amount							$ 30,000,000.0
Mortgages [Member] | First Stamford Place [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 180,000,000							235,067,000
Stated Rate	4.28%				4.28%
Effective rate	4.60%
Face amount					$ 180,000,000.0
Mortgages [Member] | First Stamford Place - First Lien [Member]
Debt Instrument [Line Items]
Stated Rate	4.09%				4.09%
Face amount	$ 164,000,000				$ 164,000,000.0
Mortgages [Member] | First Stamford Place - Second Lien [Member]
Debt Instrument [Line Items]
Stated Rate	6.25%				6.25%
Face amount	$ 16,000,000				$ 16,000,000.0
Mortgages [Member] | 1010 Third Avenue and 77 West 55th Street [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 39,710,000							26,502,000
Stated Rate	4.01%				4.01%
Effective rate	4.39%
Face amount					$ 40,000,000.0
Mortgages [Member] | 383 Main Avenue [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 30,000,000							28,654,000
Stated Rate	4.44%				4.44%
Effective rate	4.72%
Face amount					$ 30,000,000.0
Convertible debt [Member] | Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 250,000,000							250,000,000
Stated Rate	2.625%								2.625%
Effective rate	3.93%								3.80%
Face amount									$ 250,000,000
Senior Notes [Member] | Series A Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Total	$ 100,000,000							100,000,000
Stated Rate	3.93%
Effective rate	3.98%
Senior Notes [Member] | Series B Senior Notes due 2027 [Member]
Debt Instrument [Line Items]
Total	$ 125,000,000							125,000,000
Stated Rate	4.09%
Effective rate	4.14%
Senior Notes [Member] | Series C Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Total	$ 125,000,000							125,000,000
Stated Rate	4.18%
Effective rate	4.23%
Senior Notes [Member] | Series D Senior Notes due 2028 [Member]
Debt Instrument [Line Items]
Total	$ 115,000,000							0
Stated Rate	4.08%
Effective rate	4.08%
Face amount	$ 115,000,000.0
Senior Notes [Member] | Series E Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Total	$ 0							0
Stated Rate	4.26%
Effective rate	0.00%
Face amount	$ 160,000,000.0
Senior Notes [Member] | Series F Senior Notes due 2033 [Member]
Debt Instrument [Line Items]
Total	$ 0							0
Stated Rate	4.44%
Effective rate	0.00%
Face amount	$ 175,000,000.0
Revolving Credit Facility [Member] | Unsecured revolving credit facility [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0							0
Effective rate	2.66%
Revolving Credit Facility [Member] | Unsecured revolving credit facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.10%
Revolving Credit Facility [Member] | Unsecured Term Loan Facility [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 265,000,000			$ 265,000,000				$ 265,000,000
Effective rate	2.1485%
Pay rate	3.35%
Revolving Credit Facility [Member] | Unsecured Term Loan Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.20%
Subsequent Event [Member] | Mortgages [Member] | 1333 Broadway [Member]
Debt Instrument [Line Items]
Stated Rate			4.21%
Face amount			$ 160,000,000
Scenario, Forecast [Member] | Subsequent Event [Member] | Senior Notes [Member] | Series E Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Face amount		$ 160,000,000
Scenario, Forecast [Member] | Subsequent Event [Member] | Senior Notes [Member] | Series F Senior Notes due 2033 [Member]
Debt Instrument [Line Items]
Face amount		$ 175,000,000