Preferred Equity Investments - Parenthetical (Details) ft² in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) ft²
Schedule Of Investments [Line Items]
Area of office properties	12,500
470 Vanderbilt Avenue [Member]
Schedule Of Investments [Line Items]
Preferred equity investments, partnership amount | $	$ 33,750
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]
Preferred equity investments, partnership amount | $	$ 17,500
Area of office properties	369
Preferred Equity Investments, Dividend Rate	10.30%
Preferred equity investments, dividend rate percentage paid	7.00%