Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail) - Level 3 Inputs	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Minimum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.66%	0.66%
Maximum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.66%	0.90%
Weighted- Average | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.66%	0.66%
Measurement Input, Discount Rate | Minimum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.00%	1.90%
Measurement Input, Discount Rate | Maximum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	15.60%	17.10%
Measurement Input, Discount Rate | Weighted- Average | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.10%	8.80%
Measurement Input, Discount Rate | Loan Trailing Fee Liability | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.00%	1.90%
Measurement Input, Discount Rate | Loan Trailing Fee Liability | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	15.60%	17.10%
Measurement Input, Discount Rate | Loan Trailing Fee Liability | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.60%	8.90%
Measurement Input, Discount Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	3.30%	2.90%
Measurement Input, Discount Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	17.20%	17.20%
Measurement Input, Discount Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.20%	8.40%
Measurement Input, Loss Severity | Minimum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	0.40%	0.40%
Measurement Input, Loss Severity | Maximum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	42.10%	41.80%
Measurement Input, Loss Severity | Weighted- Average | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	12.20%	12.40%
Measurement Input, Loss Severity | Loan Trailing Fee Liability | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	1.00%	0.80%
Measurement Input, Loss Severity | Loan Trailing Fee Liability | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	42.10%	41.80%
Measurement Input, Loss Severity | Loan Trailing Fee Liability | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	13.30%	13.20%
Measurement Input, Loss Severity | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	0.40%	0.40%
Measurement Input, Loss Severity | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	42.10%	41.80%
Measurement Input, Loss Severity | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	12.80%	13.70%
Measurement Input, Prepayment Rate | Minimum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	11.30%	11.30%
Measurement Input, Prepayment Rate | Maximum | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	46.10%	51.00%
Measurement Input, Prepayment Rate | Weighted- Average | Servicing Assets and Servicing Liabilities
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	32.20%	31.70%
Measurement Input, Prepayment Rate | Loan Trailing Fee Liability | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	11.30%	11.30%
Measurement Input, Prepayment Rate | Loan Trailing Fee Liability | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	46.10%	51.00%
Measurement Input, Prepayment Rate | Loan Trailing Fee Liability | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	32.30%	31.40%
Measurement Input, Prepayment Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	21.30%	13.50%
Measurement Input, Prepayment Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	46.10%	51.00%
Measurement Input, Prepayment Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	31.00%	31.30%
Loans Invested in by Company | Measurement Input, Discount Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	0.90%	1.70%
Loans Invested in by Company | Measurement Input, Discount Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	17.20%	17.20%
Loans Invested in by Company | Measurement Input, Discount Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.10%	9.30%
Loans Invested in by Company | Measurement Input, Loss Severity | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	1.90%	0.80%
Loans Invested in by Company | Measurement Input, Loss Severity | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	42.10%	41.80%
Loans Invested in by Company | Measurement Input, Loss Severity | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	15.00%	14.30%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	11.30%	11.30%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	46.00%	46.00%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	32.70%	33.30%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Discount Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	3.20%	5.80%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Discount Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	15.00%	15.00%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Discount Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.80%	9.50%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Loss Severity | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	6.60%	10.90%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Loss Severity | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	43.70%	37.20%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Loss Severity | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	17.00%	19.70%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Prepayment Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	24.40%	28.30%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Prepayment Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	32.50%	33.70%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Prepayment Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.20%	30.50%