Preferred Equity Investments - Parenthetical (Details) ft² in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)
Schedule Of Investments [Line Items]
Area of office properties	12,500
Valuation allowance on preferred equity investment | $	$ 0	$ 19,588
470 Vanderbilt Avenue [Member]
Schedule Of Investments [Line Items]
Area of office properties	686
Preferred Equity Investments, Dividend Rate	10.30%
Preferred equity investments, dividend rate percentage paid	8.00%
2 Herald Square [Member]
Schedule Of Investments [Line Items]
Area of office properties	369
Valuation allowance on preferred equity investment | $			$ 19,588