Acquisition (Details)		6 Months Ended
	Feb. 09, 2018 USD ($) ft²	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Business Acquisition [Line Items]
Distributions of capital from partially owned entities | $		$ 56,500,000	$ 11,841,000
537 West 26th Street
Business Acquisition [Line Items]
Area of land (in sq feet) | ft²	14,000
Area of additional zoning air rights (in sq feet) | ft²	55,000
Distributions of capital from partially owned entities | $	$ 44,000,000