Fair Value Measurements (Narrative) (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) investment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) investment
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	$ 17,453,000		$ 17,453,000		$ 222,649,000
Non-cash impairment loss			$ 0	$ 26,140,000
Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Number of investments held by fund (investment) | investment			4		4
Real estate fund investments	17,453,000		$ 17,453,000		$ 222,649,000
Excess of fair value over cost	324,111,000		324,111,000
Level 3 | Real estate fund investments
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate fund investments	17,453,000		17,453,000
Excess of fair value over cost	324,111,000		324,111,000
Nonrecurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Real estate asset					$ 0
Fifth Avenue and Times Square JV
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Non-cash impairment loss	306,326,000	$ 0	$ 306,326,000	$ 0
Non-cash impairment loss related to noncontrolling interest	467,000
Fifth Avenue and Times Square JV | Nonrecurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Non-cash impairment loss	306,326,000
Non-cash impairment loss related to noncontrolling interest	$ 467,000
Cap Rate | Fifth Avenue and Times Square JV | Nonrecurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other real estate owned, measurement input	0.045		0.045
Discount Rate | Fifth Avenue and Times Square JV | Nonrecurring
Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other real estate owned, measurement input	0.060		0.060