Loans and Investments - Non-performing Loans (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($) loan
Non-performing loans by asset class
Number of loans for which no provision for loan loss made | loan	0	0
Number of loans | loan	476	328
Amortized cost of loans on nonaccrual status	$ 0	$ 0
UPB	$ 168,056	$ 282,785
Non-performing loans
Non-performing loans by asset class
Number of loans | loan	6	7
Carrying value of loans	$ 53,000	$ 53,800
Loan loss reserves	2,600	6,500
UPB	57,700	62,325
Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	0	0
Greater than 90 Days Past Due
Non-performing loans by asset class
Past Due	0	0
Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	57,700	62,325
Student Housing | Non-performing loans
Non-performing loans by asset class
UPB	36,500	36,500
Student Housing | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	0	0
Student Housing | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	36,500	36,500
Multifamily | Non-performing loans
Non-performing loans by asset class
UPB	17,700	17,700
Multifamily | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	0	0
Multifamily | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	17,700	17,700
Healthcare
Non-performing loans by asset class
UPB		4,625
Healthcare | Non-performing loans
Non-performing loans by asset class
UPB		4,625
Healthcare | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	0	0
Healthcare | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due		4,625
Retail
Non-performing loans by asset class
UPB	30,045	30,079
Retail | Non-performing loans
Non-performing loans by asset class
UPB	920	920
Retail | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	0	0
Retail | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	920	920
Commercial
Non-performing loans by asset class
UPB	1,700	1,700
Commercial | Non-performing loans
Non-performing loans by asset class
UPB	1,700	1,700
Commercial | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	0	0
Commercial | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	1,700	1,700
Office
Non-performing loans by asset class
UPB	2,096	2,166
Office | Non-performing loans
Non-performing loans by asset class
UPB	880	880
Office | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	0	0
Office | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	$ 880	$ 880