Investments in Unconsolidated Joint Ventures - Summary of Financial Information (Details) - 712 Fifth Avenue [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Real estate, net	$ 199,731		$ 199,731		$ 202,040
Other assets	59,879		59,879		58,034
Total assets	259,610		259,610		260,074
Notes and mortgages payable, net	296,337		296,337		296,132
Other liabilities	4,582		4,582		4,615
Total liabilities	300,919		300,919		300,747
Partners’ deficit	(41,309)		(41,309)		(40,673)
Total liabilities and partners’ deficit	259,610		259,610		$ 260,074
Rental income	12,329	$ 12,713	24,658	$ 25,658
Tenant reimbursement income	1,245	1,209	2,590	2,517
Fee and other income	159	468	617	594
Total revenues	13,733	14,390	27,865	28,769
Operating expenses	6,126	6,102	12,310	12,068
Depreciation and amortization	2,898	3,075	5,924	5,995
Total expenses	9,024	9,177	18,234	18,063
Operating income	4,709	5,213	9,631	10,706
Interest and other income, net	157	48	270	72
Interest and debt expense	(2,674)	(3,126)	(5,319)	(5,951)
Unrealized gain on interest rate swaps		728		1,896
Net income	$ 2,192	$ 2,863	$ 4,582	$ 6,723