Concentration of Credit Risk - Summary of Loans Held for Investment Portfolio by Property Type (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%
Office
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 2,216,171	$ 2,265,187
Unfunded commitment	$ 165,191	$ 178,878
% of loan commitment	39.60%	41.90%
Loan UPB	$ 2,050,980	$ 2,086,309
% of loan UPB	40.00%	42.40%
Multifamily
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 1,715,643	$ 1,595,643
Unfunded commitment	$ 113,372	$ 121,211
% of loan commitment	30.70%	29.50%
Loan UPB	$ 1,602,571	$ 1,474,731
% of loan UPB	31.30%	30.00%
Hotel
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 658,942	$ 658,943
Unfunded commitment	$ 4,000	$ 4,000
% of loan commitment	11.80%	12.20%
Loan UPB	$ 657,358	$ 657,672
% of loan UPB	12.80%	13.40%
Life Science
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 494,600	$ 494,600
Unfunded commitment	$ 156,939	$ 163,860
% of loan commitment	8.80%	9.10%
Loan UPB	$ 337,661	$ 330,740
% of loan UPB	6.60%	6.70%
Mixed Use
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 347,409	$ 347,408
Unfunded commitment	$ 15,230	$ 17,681
% of loan commitment	6.20%	6.40%
Loan UPB	$ 332,179	$ 329,728
% of loan UPB	6.50%	6.70%
Industrial
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 113,000
Unfunded commitment	$ 6,167
% of loan commitment	2.00%
Loan UPB	$ 106,833
% of loan UPB	2.10%
Retail
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 33,000	$ 33,000
Unfunded commitment	$ 2,143	$ 2,143
% of loan commitment	0.60%	0.60%
Loan UPB	$ 23,000	$ 23,000
% of loan UPB	0.40%	0.50%
Condominium
Loans And Leases Receivable Disclosure [Line Items]
Loan commitment	$ 14,585	$ 17,163
% of loan commitment	0.30%	0.30%
Loan UPB	$ 14,585	$ 17,163
% of loan UPB	0.30%	0.30%