Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail) - Level 3 Inputs	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loan Trailing Fee Liability | Minimum
Fair Value Inputs [Abstract]
Discount rates	1.90%	3.40%
Net cumulative expected loss rates	0.80%	0.30%
Cumulative prepayment rates	11.30%	8.00%
Loan Trailing Fee Liability | Maximum
Fair Value Inputs [Abstract]
Discount rates	17.10%	15.00%
Net cumulative expected loss rates	41.80%	33.90%
Cumulative prepayment rates	51.00%	42.70%
Loan Trailing Fee Liability | Weighted Average
Fair Value Inputs [Abstract]
Discount rates	8.90%	7.70%
Net cumulative expected loss rates	13.20%	13.50%
Cumulative prepayment rates	31.40%	28.30%
Servicing Asset/Liability | Minimum
Fair Value Inputs [Abstract]
Discount rates	1.90%	3.40%
Net cumulative expected loss rates	0.40%	0.30%
Cumulative prepayment rates	11.30%	8.00%
Base market servicing rates (% per annum on unpaid principal balance)	0.66%	0.63%
Servicing Asset/Liability | Maximum
Fair Value Inputs [Abstract]
Discount rates	17.10%	15.10%
Net cumulative expected loss rates	41.80%	33.90%
Cumulative prepayment rates	51.00%	42.70%
Base market servicing rates (% per annum on unpaid principal balance)	0.90%	0.90%
Servicing Asset/Liability | Weighted Average
Fair Value Inputs [Abstract]
Discount rates	8.80%	7.80%
Net cumulative expected loss rates	12.40%	12.80%
Cumulative prepayment rates	31.70%	29.30%
Base market servicing rates (% per annum on unpaid principal balance)	0.66%	0.63%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Minimum
Fair Value Inputs [Abstract]
Discount rates	1.70%
Net cumulative expected loss rates	0.40%
Cumulative prepayment rates	11.30%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Maximum
Fair Value Inputs [Abstract]
Discount rates	17.20%
Net cumulative expected loss rates	41.80%
Cumulative prepayment rates	51.00%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Weighted Average
Fair Value Inputs [Abstract]
Discount rates	8.50%
Net cumulative expected loss rates	13.80%
Cumulative prepayment rates	31.60%
Asset-Backed Securities Related to Consolidated VIEs | Minimum
Fair Value Inputs [Abstract]
Discount rates	5.80%
Net cumulative expected loss rates	10.90%
Cumulative prepayment rates	28.30%
Asset-Backed Securities Related to Consolidated VIEs | Maximum
Fair Value Inputs [Abstract]
Discount rates	15.00%
Net cumulative expected loss rates	37.20%
Cumulative prepayment rates	33.70%
Asset-Backed Securities Related to Consolidated VIEs | Weighted Average
Fair Value Inputs [Abstract]
Discount rates	9.50%
Net cumulative expected loss rates	19.70%
Cumulative prepayment rates	30.50%
Loans Held for Investment, Loans Held for Sale, Notes and Certificates (1) | Minimum
Fair Value Inputs [Abstract]
Discount rates		1.20%
Net cumulative expected loss rates		0.30%
Cumulative prepayment rates		8.00%
Loans Held for Investment, Loans Held for Sale, Notes and Certificates (1) | Maximum
Fair Value Inputs [Abstract]
Discount rates		16.60%
Net cumulative expected loss rates		33.90%
Cumulative prepayment rates		42.70%
Loans Held for Investment, Loans Held for Sale, Notes and Certificates (1) | Weighted Average
Fair Value Inputs [Abstract]
Discount rates		7.20%
Net cumulative expected loss rates		14.60%
Cumulative prepayment rates		30.70%