Concentration of Credit Risk - Summary of Geographic Composition of Loans Held for Investment Based on Current UPB and Loan Commitment (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 5,593,350	$ 5,411,944
Unfunded commitment	$ 463,042	$ 487,773
% of loan commitment	100.00%	100.00%
Loan UPB	$ 5,125,167	$ 4,919,343
% of loan UPB	100.00%	100.00%
East
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 2,058,483	$ 1,925,457
Unfunded commitment	$ 50,628	$ 63,459
% of loan commitment	36.80%	35.60%
Loan UPB	$ 2,009,028	$ 1,863,172
% of loan UPB	39.20%	37.80%
West
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 1,503,608	$ 1,484,883
Unfunded commitment	$ 235,822	$ 244,100
% of loan commitment	26.90%	27.40%
Loan UPB	$ 1,260,631	$ 1,233,628
% of loan UPB	24.60%	25.10%
South
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 1,283,455	$ 1,323,800
Unfunded commitment	$ 110,144	$ 115,714
% of loan commitment	22.90%	24.50%
Loan UPB	$ 1,173,852	$ 1,208,940
% of loan UPB	22.90%	24.60%
Midwest
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 677,804	$ 677,804
Unfunded commitment	$ 60,281	$ 64,500
% of loan commitment	12.10%	12.50%
Loan UPB	$ 617,823	$ 613,603
% of loan UPB	12.10%	12.50%
Various
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 70,000
Unfunded commitment	$ 6,167
% of loan commitment	1.30%
Loan UPB	$ 63,833
% of loan UPB	1.20%