Concentration of Credit Risk - Summary of Geographic Composition of Loans Held for Investment Based on Current UPB and Loan Commitment (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 5,318,297	$ 4,943,511
Unfunded Commitment	$ 488,916	$ 423,487
% Loan Commitment	100.00%	100.00%
Loan UPB	$ 4,833,535	$ 4,524,725
% of Loan UPB	100.00%	100.00%
East
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 2,101,475	$ 2,009,022
Unfunded Commitment	$ 126,872	$ 152,487
% Loan Commitment	39.50%	40.60%
Loan UPB	$ 1,975,566	$ 1,856,535
% of Loan UPB	40.90%	41.00%
South
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,358,016	$ 1,289,141
Unfunded Commitment	$ 93,438	$ 97,405
% Loan Commitment	25.50%	26.10%
Loan UPB	$ 1,265,693	$ 1,192,852
% of Loan UPB	26.20%	26.40%
West
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,207,005	$ 1,128,897
Unfunded Commitment	$ 202,935	$ 121,738
% Loan Commitment	22.70%	22.80%
Loan UPB	$ 1,005,804	$ 1,009,589
% of Loan UPB	20.80%	22.30%
Midwest
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 563,701	$ 428,351
Unfunded Commitment	$ 64,628	$ 49,478
% Loan Commitment	10.60%	8.70%
Loan UPB	$ 499,374	$ 379,173
% of Loan UPB	10.30%	8.40%
Various
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 88,100	$ 88,100
Unfunded Commitment	$ 1,043	$ 2,379
% Loan Commitment	1.70%	1.80%
Loan UPB	$ 87,098	$ 86,576
% of Loan UPB	1.80%	1.90%