Concentration of Credit Risk - Summary of Geographic Composition of Loans Held for Investment Based on Current UPB and Loan Commitment (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%
East
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 2,070,677	$ 2,084,807
Unfunded Commitment	$ 159,512	$ 170,131
% Loan Commitment	38.60%	42.10%
Loan UPB	$ 1,911,165	$ 1,914,676
% Loan UPB	40.50%	44.40%
South
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,586,916	$ 1,525,173
Unfunded Commitment	$ 259,802	$ 270,933
% Loan Commitment	29.50%	30.80%
Loan UPB	$ 1,327,114	$ 1,254,240
% Loan UPB	28.00%	29.10%
West
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 902,274	$ 760,416
Unfunded Commitment	$ 77,586	$ 100,422
% Loan Commitment	16.80%	15.40%
Loan UPB	$ 824,688	$ 659,994
% Loan UPB	17.40%	15.30%
Midwest
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 725,704	$ 577,353
Unfunded Commitment	$ 122,509	$ 92,672
% Loan Commitment	13.50%	11.70%
Loan UPB	$ 603,195	$ 484,681
% Loan UPB	12.70%	11.20%
Various
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 88,100
Unfunded Commitment	$ 21,300
% Loan Commitment	1.60%
Loan UPB	$ 66,800
% Loan UPB	1.40%