Concentration of Credit Risk - Summary of Loans Held for Investment Portfolio by Loan Category Type (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 5,240,569	$ 4,943,511
Unfunded commitment	$ 486,663	$ 423,487
% of loan commitment	100.00%	100.00%
Loan UPB	$ 4,757,397	$ 4,524,725
% of loan UPB	100.00%	100.00%
Light Transitional
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 1,700,225	$ 1,937,644
Unfunded commitment	$ 129,504	$ 173,518
% of loan commitment	32.40%	39.20%
Loan UPB	$ 1,570,720	$ 1,764,126
% of loan UPB	33.00%	39.00%
Moderate Transitional
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 1,945,826	$ 1,633,131
Unfunded commitment	$ 326,944	$ 224,532
% of loan commitment	37.10%	33.00%
Loan UPB	$ 1,620,054	$ 1,410,145
% of loan UPB	34.10%	31.20%
Bridge
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 1,559,518	$ 1,337,736
Unfunded commitment	$ 30,215	$ 22,953
% of loan commitment	29.80%	27.10%
Loan UPB	$ 1,531,623	$ 1,317,938
% of loan UPB	32.20%	29.10%
Construction
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 35,000	$ 35,000
Unfunded commitment		$ 2,484
% of loan commitment	0.70%	0.70%
Loan UPB	$ 35,000	$ 32,516
% of loan UPB	0.70%	0.70%