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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Real estate, net	$ 204,083		$ 204,083		$ 207,632
Other assets	56,710		56,710		40,701
Total assets	260,793		260,793		248,333
Notes and mortgages payable, net	296,051		296,051		245,990
Other liabilities	5,765		5,765		8,783
Total liabilities	301,816		301,816		254,773
Equity	(41,023)		(41,023)		(6,440)
Total liabilities and equity	260,793		260,793		$ 248,333
Rental income	12,626	$ 12,107	38,284	$ 37,501
Tenant reimbursement income	1,338	1,342	3,855	3,351
Fee and other income	507	418	1,101	1,613
Total revenues	14,471	13,867	43,240	42,465
Operating expenses	6,197	6,081	18,265	17,073
Depreciation and amortization	3,067	3,193	9,062	9,244
Total expenses	9,264	9,274	27,327	26,317
Operating income	5,207	4,593	15,913	16,148
Interest and other income, net	68	16	140	49
Interest and debt expense	(2,700)	(2,787)	(8,651)	(8,287)
Unrealized gain on interest rate swaps		1,722	1,896	2,556
Net income	$ 2,575	$ 3,544	$ 9,298	$ 10,466