Debt and Preferred Equity Investments - Rollforward of Total Allowance for Loan Loss Reserves (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Loan loss reserve activity
Balance at beginning of year			$ 13,213,000	$ 1,750,000	$ 1,750,000
Current period provision for loan loss	$ 0	$ 0	0	6,300,000	20,693,000
Write-offs charged against the allowance			(6,583,000)		(37,033,000)
Balance at end of period	$ 6,630,000		$ 6,630,000		13,213,000
Number of financing receivables on non-accrual without allowance for loan loss | loan	1		1
Financing receivable on non-accrual without allowance	$ 225,400,000		$ 225,400,000
Investments Sold In 2020
Loan loss reserve activity
Write-offs charged against the allowance			(300,000)		(19,000,000)
Cumulative Effect, Period of Adoption, Adjustment
Loan loss reserve activity
Balance at beginning of year			$ 0	$ 27,803,000	27,803,000
Balance at end of period					$ 0