Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Assets And Liabilities Measured At Fair Value
Marketable securities	$ 41,119	$ 44,646
Short-term Investments	0	24,998
Interest rate cap (included in other assets)		11
Total assets	41,119	69,655
Liabilities measured at fair value	0	0
Carrying Reported Amount Fair Value Disclosure [Member]
Assets
Cash Equivalents	221,539	111,590
Liabilities
Mortgages Payable	1,060,408	1,032,780
Leasing commissions (included in amounts due to Vornado)	15	0
Total liabilities not measured at fair value	1,060,423	1,032,780
Estimate Of Fair Value Fair Value Disclosure [Member]
Assets
Cash Equivalents	221,539	111,590
Liabilities
Mortgages Payable	1,054,200	1,025,000
Leasing commissions (included in amounts due to Vornado)	15	0
Total liabilities not measured at fair value	1,054,215	1,025,000
Fair Value, Inputs, Level 1 [Member]
Financial Assets And Liabilities Measured At Fair Value
Marketable securities	41,119	44,646
Short-term Investments		24,998
Interest rate cap (included in other assets)		0
Total assets	41,119	69,644
Fair Value, Inputs, Level 2 [Member]
Financial Assets And Liabilities Measured At Fair Value
Marketable securities	0	0
Short-term Investments		0
Interest rate cap (included in other assets)		11
Total assets	0	11
Fair Value, Inputs, Level 3 [Member]
Financial Assets And Liabilities Measured At Fair Value
Marketable securities	0	0
Short-term Investments		0
Interest rate cap (included in other assets)		0
Total assets	$ 0	$ 0