Concentration of Credit Risk - Summary of Loan Portfolio by Property Type (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 5,373,671	$ 4,947,749
Unfunded Commitment	$ 640,709	$ 634,158
% of Loan Commitment	100.00%	100.00%
Loan UPB	$ 4,732,962	$ 4,313,591
% of Loan UPB	100.00%	100.00%
Office
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,926,459	$ 1,898,511
Unfunded Commitment	$ 312,218	$ 316,510
% of Loan Commitment	35.90%	38.50%
Loan UPB	$ 1,614,241	$ 1,582,001
% of Loan UPB	34.20%	36.80%
Multifamily
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,384,512	$ 1,247,860
Unfunded Commitment	$ 129,447	$ 131,177
% of Loan Commitment	25.80%	25.20%
Loan UPB	$ 1,255,065	$ 1,116,683
% of Loan UPB	26.50%	25.90%
Mixed Use
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 919,485	$ 838,200
Unfunded Commitment	$ 107,801	$ 114,748
% of Loan Commitment	17.10%	16.90%
Loan UPB	$ 811,684	$ 723,452
% of Loan UPB	17.10%	16.80%
Hotel
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 667,793	$ 508,450
Unfunded Commitment	$ 37,188	$ 10,896
% of Loan Commitment	12.40%	10.30%
Loan UPB	$ 630,605	$ 497,554
% of Loan UPB	13.30%	11.50%
Retail
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 233,554	$ 233,555
Unfunded Commitment	$ 48,844	$ 50,247
% of Loan Commitment	4.30%	4.70%
Loan UPB	$ 184,710	$ 183,308
% of Loan UPB	3.90%	4.20%
Condominium
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 129,868	$ 154,673
Unfunded Commitment	$ 5,211	$ 10,580
% of Loan Commitment	2.40%	3.10%
Loan UPB	$ 124,657	$ 144,093
% of Loan UPB	2.60%	3.30%
Industrial
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment		$ 66,500
% of Loan Commitment		1.30%
Loan UPB		$ 66,500
% of Loan UPB		1.50%
Other
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 112,000
% of Loan Commitment	2.10%
Loan UPB	$ 112,000
% of Loan UPB	2.40%