Investments in Unconsolidated Joint Ventures - Summary of Financial Information (Details) - 712 Fifth Avenue [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Real estate, net	$ 199,948		$ 202,040
Other assets	62,989		58,034
Total assets	262,937		260,074
Notes and mortgages payable, net	296,235		296,132
Other liabilities	4,986		4,615
Total liabilities	301,221		300,747
Equity	(38,284)		(40,673)
Total liabilities and equity	262,937		$ 260,074
Rental income	12,329	$ 12,945
Tenant reimbursement income	1,345	1,308
Fee and other income	458	126
Total revenues	14,132	14,379
Operating expenses	6,184	5,966
Depreciation and amortization	3,026	2,920
Total expenses	9,210	8,886
Operating income	4,922	5,493
Interest and other income, net	113	24
Interest and debt expense	(2,645)	(2,825)
Unrealized gain on interest rate swaps		1,168
Net income	$ 2,390	$ 3,860