Loans and Investments - Non-performing Loans (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) loan item	Dec. 31, 2019 USD ($) loan item
Non-performing loans by asset class
Number of loans | loan	328	272
Number of impaired loans (less unearned revenue and other holdbacks and adjustments) by asset class | item	10	5
Carrying value of loans	$ 5,475,082,000
Amortized cost of loans on nonaccrual status	0	$ 0
UPB	$ 282,785,000	$ 138,141,000
Non-performing loans
Non-performing loans by asset class
Number of loans | loan	7	3
Carrying value of loans	$ 53,800,000	$ 1,800,000
Loan loss reserves	6,500,000	1,700,000
UPB	62,325,000	3,580,000
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	62,325,000	3,580,000
Interest Receivable	0	0
Student Housing | Non-performing loans
Non-performing loans by asset class
UPB	36,500,000
Student Housing | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	36,500,000
Multifamily | Non-performing loans
Non-performing loans by asset class
UPB	17,700,000
Multifamily | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	17,700,000
Healthcare
Non-performing loans by asset class
UPB	4,625,000
Healthcare | Non-performing loans
Non-performing loans by asset class
UPB	4,625,000
Healthcare | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	4,625,000
Retail
Non-performing loans by asset class
UPB	30,079,000
Retail | Non-performing loans
Non-performing loans by asset class
UPB	920,000	1,000,000
Retail | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	920,000	1,000,000
Commercial
Non-performing loans by asset class
UPB	1,700,000	1,700,000
Commercial | Non-performing loans
Non-performing loans by asset class
UPB	1,700,000	1,700,000
Commercial | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	1,700,000	1,700,000
Office
Non-performing loans by asset class
UPB	2,166,000	2,226,000
Office | Non-performing loans
Non-performing loans by asset class
UPB	880,000	880,000
Office | Greater than 90 Days Past Due | Non-performing loans
Non-performing loans by asset class
Past Due	$ 880,000	$ 880,000