Investments in Unconsolidated Joint Ventures - Summary of Financial Information (Details) - 712 Fifth Avenue [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Real estate, net	$ 199,862		$ 199,862		$ 202,040
Other assets	61,283		61,283		58,034
Total assets	261,145		261,145		260,074
Notes and mortgages payable, net	296,440		296,440		296,132
Other liabilities	5,217		5,217		4,615
Total liabilities	301,657		301,657		300,747
Partners’ deficit	(40,512)		(40,512)		(40,673)
Total liabilities and partners’ deficit	261,145		261,145		$ 260,074
Rental income	12,229	$ 12,626	36,887	$ 38,284
Tenant reimbursement income	1,227	1,338	3,817	3,855
Fee and other income	125	507	742	1,101
Total revenues	13,581	14,471	41,446	43,240
Operating expenses	6,250	6,197	18,560	18,265
Depreciation and amortization	2,864	3,067	8,788	9,062
Total expenses	9,114	9,264	27,348	27,327
Operating income	4,467	5,207	14,098	15,913
Interest and other income, net	146	68	416	140
Interest and debt expense	(2,701)	(2,700)	(8,020)	(8,651)
Unrealized gain on interest rate swaps				1,896
Net income	$ 1,912	$ 2,575	$ 6,494	$ 9,298