Concentration of Credit Risk - Summary of Loans Held for Investment Portfolio by Property 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%
Office
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 2,778,523	$ 2,756,338
Unfunded commitment	$ 386,673	$ 356,034
% of loan commitment	53.10%	55.70%
Loan UPB	$ 2,391,850	$ 2,400,304
% of loan UPB	50.20%	53.00%
Multifamily
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 1,249,025	$ 804,838
Unfunded commitment	$ 68,246	$ 24,001
% of loan commitment	23.80%	16.30%
Loan UPB	$ 1,181,078	$ 781,137
% of loan UPB	24.80%	17.30%
Hotel
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 584,943	$ 737,293
Unfunded commitment	$ 5,000	$ 9,864
% of loan commitment	11.20%	14.90%
Loan UPB	$ 582,792	$ 731,487
% of loan UPB	12.30%	16.20%
Mixed Use
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 572,069	$ 586,993
Unfunded commitment	$ 24,601	$ 31,398
% of loan commitment	10.90%	11.90%
Loan UPB	$ 547,468	$ 555,595
% of loan UPB	11.50%	12.30%
Condominium
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 23,009	$ 25,049
% of loan commitment	0.40%	0.50%
Loan UPB	$ 23,009	$ 25,049
% of loan UPB	0.50%	0.60%
Retail
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 33,000	$ 33,000
Unfunded commitment	$ 2,143	$ 2,190
% of loan commitment	0.60%	0.70%
Loan UPB	$ 31,200	$ 31,153
% of loan UPB	0.70%	0.70%