Related Party Transactions (Fees to Vornado) (Details) - Vornado - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of fees to Vornado
Fees to related party	$ 8,772	$ 14,428	$ 11,799
Company management fees
Summary of fees to Vornado
Fees to related party	2,800	2,800	2,800
Development fees
Summary of fees to Vornado
Fees to related party	29	194	2,435
Leasing fees
Summary of fees to Vornado
Fees to related party	1,829	7,401	2,950
Property management fees and payments for cleaning, engineering and security services
Summary of fees to Vornado
Fees to related party	$ 4,114	$ 4,033	$ 3,614