Concentration of Credit Risk - Summary of Geographic Composition of Loans Held for Investment Based on Current UPB and Loan Commitment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 4,705,831	$ 3,727,156
Unfunded Commitment	$ 527,102	$ 529,040
% Loan Commitment	100.00%	100.00%
Loan UPB	$ 4,178,729	$ 3,198,116
% Loan UPB	100.00%	100.00%
East
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 2,068,065	$ 1,600,619
Unfunded Commitment	$ 163,056	$ 167,447
% Loan Commitment	44.00%	42.90%
Loan UPB	$ 1,905,009	$ 1,433,172
% Loan UPB	45.60%	44.80%
South
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 1,442,929	$ 1,147,510
Unfunded Commitment	$ 257,013	$ 278,890
% Loan Commitment	30.70%	30.80%
Loan UPB	$ 1,185,916	$ 868,620
% Loan UPB	28.40%	27.20%
West
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 744,940	$ 674,123
Unfunded Commitment	$ 87,885	$ 67,746
% Loan Commitment	15.80%	18.10%
Loan UPB	$ 657,055	$ 606,377
% Loan UPB	15.70%	19.00%
Midwest
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 400,897	$ 255,904
Unfunded Commitment	$ 19,148	$ 14,957
% Loan Commitment	8.50%	6.90%
Loan UPB	$ 381,749	$ 240,947
% Loan UPB	9.10%	7.50%
Various
Loans And Leases Receivable Disclosure [Line Items]
Loan Commitment	$ 49,000	$ 49,000
% Loan Commitment	1.00%	1.30%
Loan UPB	$ 49,000	$ 49,000
% Loan UPB	1.20%	1.50%