Loans Held for Investment and the Allowance for Credit Losses - Summary of Activity in Allowance for Credit Losses for Mortgage Loan Investment Portfolio by Class of Financing Receivable (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accounts Notes And Loans Receivable [Line Items]
Beginning balance			$ 59,940
Allowance for credit losses for loans held for investment	$ 51,941		51,941
Total allowance for credit losses	55,312	$ 58,675	55,312	$ 58,675
Loans Held for Investment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	56,641	75,658	59,940
Credit Loss Expense (Benefit)	(4,700)	(22,101)	(7,999)	35,774
Allowance for credit losses for loans held for investment	51,941	53,557	51,941	53,557
Loans Held for Investment | ASU 2016-13
Accounts Notes And Loans Receivable [Line Items]
Cumulative-effect adjustment upon adoption of ASU 2016-13				17,783
Unfunded Loan Commitment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	2,149	7,335	2,888
Credit Loss Expense (Benefit)	1,222	(2,217)	483	3,256
Allowance for credit losses on unfunded loan commitments	3,371	5,118	3,371	5,118
Unfunded Loan Commitment | ASU 2016-13
Accounts Notes And Loans Receivable [Line Items]
Cumulative-effect adjustment upon adoption of ASU 2016-13				1,862
Senior Loans
Accounts Notes And Loans Receivable [Line Items]
Total allowance for credit losses	54,791	55,175	54,791	55,175
Senior Loans | Loans Held for Investment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	55,155	73,620	58,210
Credit Loss Expense (Benefit)	(3,724)	(22,063)	(6,779)	34,654
Allowance for credit losses for loans held for investment	51,431	51,557	51,431	51,557
Senior Loans | Loans Held for Investment | ASU 2016-13
Accounts Notes And Loans Receivable [Line Items]
Cumulative-effect adjustment upon adoption of ASU 2016-13				16,903
Senior Loans | Unfunded Loan Commitment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	2,084	5,807	2,756
Credit Loss Expense (Benefit)	1,276	(2,189)	604	1,756
Allowance for credit losses on unfunded loan commitments	3,360	3,618	3,360	3,618
Senior Loans | Unfunded Loan Commitment | ASU 2016-13
Accounts Notes And Loans Receivable [Line Items]
Cumulative-effect adjustment upon adoption of ASU 2016-13				1,862
Subordinated and Mezzanine Loans
Accounts Notes And Loans Receivable [Line Items]
Total allowance for credit losses	521	3,500	521	3,500
Subordinated and Mezzanine Loans | Loans Held for Investment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	1,486	2,038	1,730
Credit Loss Expense (Benefit)	(976)	(38)	(1,220)	1,120
Allowance for credit losses for loans held for investment	510	2,000	510	2,000
Subordinated and Mezzanine Loans | Loans Held for Investment | ASU 2016-13
Accounts Notes And Loans Receivable [Line Items]
Cumulative-effect adjustment upon adoption of ASU 2016-13				880
Subordinated and Mezzanine Loans | Unfunded Loan Commitment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	65	1,528	132
Credit Loss Expense (Benefit)	(54)	(28)	(121)	1,500
Allowance for credit losses on unfunded loan commitments	$ 11	$ 1,500	$ 11	$ 1,500