Investments in Real Estate Entities - Expensed Acquisition, Development and Other Pursuit Costs and Impairment of Long-Lived Assets & Casualty Gains and Losses (Details) ft² in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) home	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) home ft²	Jun. 30, 2018 USD ($)
Equity method investment
Expensed transaction, development and other pursuit costs, net of recoveries	$ 2,711,000	$ 1,047,000	$ 3,806,000	$ 1,847,000
Impairment of investment in unconsolidated entities	0	0	0	0
Casualty and impairment (gain) loss, net	0	0	0	(58,000)
15 West 61st Street [Member]
Equity method investment
Expensed transaction, development and other pursuit costs, net of recoveries	945,000	158,000	1,418,000	158,000
Land Parcel [Member]
Equity method investment
Casualty and impairment (gain) loss, net	0	0	0	0
Abandoned Development Pursuits [Member]
Equity method investment
Expensed transaction, development and other pursuit costs, net of recoveries	$ 1,766,000	$ 889,000	$ 2,388,000	$ 1,689,000
Development Communities [Member] | 15 West 61st Street [Member]
Equity method investment
Number of Residential Units in Development | home	172		172
Retail Square Feet Development | ft²			67
Real Estate Investments, Projected Total Capitalized Cost	$ 624,000,000		$ 624,000,000