Loans and Investments - Non-performing Loans (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2022 USD ($) loan	Dec. 31, 2021 USD ($) 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	742	580
Amortized cost of loans on nonaccrual status	$ 0	$ 0
UPB	157,960	159,940
Loans and investments	$ 14,989,643	$ 12,158,995
Non-performing loans
Non-performing loans by asset class
Number of loans | loan	4	3
Carrying value of loans	$ 19,100	$ 20,100
Loan loss reserves	5,100	2,600
UPB	25,645	24,120
Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Loans and investments	0	0
Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Loans and investments	25,645	24,120
Student Housing | Non-performing loans
Non-performing loans by asset class
UPB	20,500	21,500
Student Housing | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Loans and investments	0	0
Student Housing | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Loans and investments	20,500	21,500
Retail
Non-performing loans by asset class
UPB	22,045	22,045
Retail | Non-performing loans
Non-performing loans by asset class
UPB	3,445	920
Retail | Less Than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Loans and investments	0	0
Retail | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Loans and investments	3,445	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
Loans and investments	0	0
Commercial | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Loans and investments	$ 1,700	$ 1,700