Loans Held for Investment and the Allowance for Credit Losses - Summary of Activity in Allowance for Credit Losses for Loans Held for Investment Portfolio by Class of Financing Receivable (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accounts Notes And Loans Receivable [Line Items]
Beginning balance			$ 59,940
Allowance for credit losses for loans held for investment	$ 50,051		50,051
Total allowance for credit losses	54,968	$ 59,329	54,968	$ 59,329
Loans Held for Investment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	51,941	53,557	59,940
Credit loss expense (benefit)	(1,890)	3,103	(9,889)	38,877
Allowance for credit losses for loans held for investment	50,051	56,660	50,051	56,660
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	3,371	5,118	2,888
Credit loss expense (benefit)	1,546	(2,449)	2,029	807
Allowance for credit losses on unfunded loan commitments	4,917	2,669	4,917	2,669
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	54,076	57,453	54,076	57,453
Senior Loans | Loans Held for Investment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	51,431	51,557	58,210
Credit loss expense (benefit)	(1,567)	3,416	(8,346)	38,070
Allowance for credit losses for loans held for investment	49,864	54,973	49,864	54,973
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	3,360	3,618	2,756
Credit loss expense (benefit)	852	(1,138)	1,456	618
Allowance for credit losses on unfunded loan commitments	4,212	2,480	4,212	2,480
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	892	1,876	892	1,876
Subordinated and Mezzanine Loans | Loans Held for Investment
Accounts Notes And Loans Receivable [Line Items]
Beginning balance	510	2,000	1,730
Credit loss expense (benefit)	(323)	(313)	(1,543)	807
Allowance for credit losses for loans held for investment	187	1,687	187	1,687
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	11	1,500	132
Credit loss expense (benefit)	694	(1,311)	573	189
Allowance for credit losses on unfunded loan commitments	$ 705	$ 189	$ 705	$ 189