Concentration of Credit Risk - Summary of Loan Portfolio by Loan Category 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
% Loan Commitment	100.00%	100.00%
Loan UPB	$ 4,732,962	$ 4,313,591
% Loan UPB	100.00%	100.00%
Bridge
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 2,661,392	$ 2,414,456
Unfunded Commitment	$ 193,047	$ 199,397
% Loan Commitment	49.50%	48.80%
Loan UPB	$ 2,468,345	$ 2,215,059
% Loan UPB	52.10%	51.30%
Light Transitional
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,460,627	$ 1,513,227
Unfunded Commitment	$ 168,838	$ 212,290
% Loan Commitment	27.20%	30.60%
Loan UPB	$ 1,291,789	$ 1,300,937
% Loan UPB	27.30%	30.20%
Moderate Transitional
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,251,652	$ 1,020,066
Unfunded Commitment	$ 278,824	$ 222,471
% Loan Commitment	23.30%	20.60%
Loan UPB	$ 972,828	$ 797,595
% Loan UPB	20.60%	18.50%