Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail) - Level 3 Inputs	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Minimum | Servicing Assets/Liabilities
Fair Value Inputs [Abstract]
Discount rates	3.40%	1.90%
Net cumulative expected loss rates	0.40%	0.40%
Cumulative prepayment rates	11.30%	11.30%
Total market servicing rates (percent per annum on outstanding principal balance)	0.66%	0.66%
Maximum | Servicing Assets/Liabilities
Fair Value Inputs [Abstract]
Discount rates	16.50%	17.10%
Net cumulative expected loss rates	42.10%	41.80%
Cumulative prepayment rates	52.30%	51.00%
Total market servicing rates (percent per annum on outstanding principal balance)	0.90%	0.90%
Weighted- Average | Servicing Assets/Liabilities
Fair Value Inputs [Abstract]
Discount rates	8.90%	8.80%
Net cumulative expected loss rates	12.30%	12.40%
Cumulative prepayment rates	32.00%	31.70%
Total market servicing rates (percent per annum on outstanding principal balance)	0.66%	0.66%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Minimum
Fair Value Inputs [Abstract]
Discount rates	2.70%	1.70%
Net cumulative expected loss rates	0.40%	0.40%
Cumulative prepayment rates	11.30%	11.30%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Maximum
Fair Value Inputs [Abstract]
Discount rates	16.70%	17.20%
Net cumulative expected loss rates	42.10%	41.80%
Cumulative prepayment rates	52.30%	51.00%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Weighted- Average
Fair Value Inputs [Abstract]
Discount rates	8.90%	8.50%
Net cumulative expected loss rates	13.80%	13.80%
Cumulative prepayment rates	31.30%	31.60%
Asset-Backed Securities Related to Consolidated VIEs | Minimum
Fair Value Inputs [Abstract]
Discount rates	3.20%	5.80%
Net cumulative expected loss rates	5.20%	10.90%
Cumulative prepayment rates	25.70%	28.30%
Asset-Backed Securities Related to Consolidated VIEs | Maximum
Fair Value Inputs [Abstract]
Discount rates	15.00%	15.00%
Net cumulative expected loss rates	38.70%	37.20%
Cumulative prepayment rates	34.20%	33.70%
Asset-Backed Securities Related to Consolidated VIEs | Weighted- Average
Fair Value Inputs [Abstract]
Discount rates	7.40%	9.50%
Net cumulative expected loss rates	13.30%	19.70%
Cumulative prepayment rates	30.30%	30.50%
Loan Trailing Fee Liability | Minimum
Fair Value Inputs [Abstract]
Discount rates	3.40%	1.90%
Net cumulative expected loss rates	0.80%	0.80%
Cumulative prepayment rates	11.30%	11.30%
Loan Trailing Fee Liability | Maximum
Fair Value Inputs [Abstract]
Discount rates	16.50%	17.10%
Net cumulative expected loss rates	42.10%	41.80%
Cumulative prepayment rates	52.30%	51.00%
Loan Trailing Fee Liability | Weighted- Average
Fair Value Inputs [Abstract]
Discount rates	9.20%	8.90%
Net cumulative expected loss rates	13.10%	13.20%
Cumulative prepayment rates	31.60%	31.40%