Preferred Equity Investments - Parenthetical (Details) ft² in Thousands	3 Months Ended
	Mar. 31, 2018 ft²
Schedule Of Investments [Line Items]
Area of office properties	12,500
470 Vanderbilt Avenue [Member]
Schedule Of Investments [Line Items]
Area of office properties	650
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
Preferred Equity Investments, Dividend Rate	10.30%