Preferred Equity Investments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Apr. 30, 2017
Investment Holdings [Line Items]
Preferred equity ownership percentage	24.40%
Preferred equity investments, gross	$ 35,925	$ 55,405
Less: valuation allowance	0	(19,588)
Total preferred equity investments, net	$ 35,925	35,817
470 Vanderbilt Avenue [Member]
Investment Holdings [Line Items]
Preferred equity ownership percentage	24.40%
Preferred Equity Investments, Dividend Rate	10.30%
Preferred Equity Investments, Initial Maturity	2019-02
Preferred equity investments, gross	$ 35,925	35,817
2 Herald Square [Member]
Investment Holdings [Line Items]
Preferred equity investments, gross		$ 19,588
Less: valuation allowance			$ (19,588)