Loans - Secured Loans in Non-Accrual Status (Details) $ in Thousands		1 Months Ended	9 Months Ended	12 Months Ended
		Jun. 30, 2020 MortgageLoan	Sep. 30, 2021 USD ($) MortgageLoan	Dec. 31, 2020 USD ($) MortgageLoan	Jun. 30, 2021 USD ($)
Loans Details Secured Loans In Nonaccrual Status [Line Items]
Number of secured loans | MortgageLoan		2	33	33
Secured loans, principal			$ 70,227	$ 74,080	$ 72,548
Accrued interest			544	716
Non-accrual Status [Member]
Loans Details Secured Loans In Nonaccrual Status [Line Items]
Secured loans, principal			6,532	15,531
Accrued interest	[1]		53	368
Foregone interest			$ 262	$ 582
Principal [Member] | Non-accrual Status [Member]
Loans Details Secured Loans In Nonaccrual Status [Line Items]
Number of secured loans | MortgageLoan			4	3
Secured loans, principal			$ 6,370	$ 15,139
Advances [Member]
Loans Details Secured Loans In Nonaccrual Status [Line Items]
Secured loans, principal				$ 24
Advances [Member] | Non-accrual Status [Member]
Loans Details Secured Loans In Nonaccrual Status [Line Items]
Secured loans, principal			$ 109

[1] Accrued interest in the table above is the amount of interest accrued prior to the loan being placed on non-accrual status, net of any payments received while in non-accrual status.