Debt and Preferred Equity Investments - Rollforward of Total Allowance for Loan Loss Reserves (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Loan loss reserve activity
Balance at beginning of year	$ 1,750	$ 5,750
Current period provision for loan loss	20,741	0
Write-offs charged against the allowance	(31,285)	(4,000)
Balance at end of period	19,010	1,750
Cumulative adjustment upon adoption of ASC 326
Loan loss reserve activity
Balance at beginning of year	$ 27,804	0
Balance at end of period		$ 27,804