Mortgages Payable (Narrative) (Details)		5 Months Ended
	Jun. 01, 2017 USD ($) option	May 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Mortgages payable, net of deferred debt issuance costs			$ 1,240,222,000	$ 1,052,359,000
731 Lexington Avenue | Office space | Mortgages
Debt Instrument [Line Items]
Mortgages payable, net of deferred debt issuance costs	$ 500,000,000	$ 300,000,000
Maturity date	Jun. 01, 2020
Number of one year extension options | option	4
Derivative liability, notional amount	$ 500,000,000
731 Lexington Avenue | Office space | Mortgages | LIBOR
Debt Instrument [Line Items]
Basis spread over LIBOR	0.90%	0.95%
Derivative, cap interest rate	6.00%