Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Loan Trailing Fee Liability | Discount rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	2.90%	4.80%
Loan Trailing Fee Liability | Discount rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	14.80%	16.70%
Loan Trailing Fee Liability | Discount rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	9.30%	9.50%
Loan Trailing Fee Liability | Net cumulative expected loss rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	3.70%	2.80%
Loan Trailing Fee Liability | Net cumulative expected loss rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	36.00%	38.70%
Loan Trailing Fee Liability | Net cumulative expected loss rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	14.40%	14.00%
Loan Trailing Fee Liability | Cumulative expected prepayment rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	28.50%	16.50%
Loan Trailing Fee Liability | Cumulative expected prepayment rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	41.70%	43.10%
Loan Trailing Fee Liability | Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	33.00%	32.20%
Servicing Assets [Member] | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair Value Inputs Market Servicing Rate	0.66%
Servicing Assets [Member] | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair Value Inputs Market Servicing Rate	0.66%	0.66%
Servicing Assets [Member] | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair Value Inputs Market Servicing Rate	0.66%	0.66%
Servicing Assets [Member] | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair Value Inputs Market Servicing Rate		0.66%
Servicing Assets [Member] | Discount rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	2.90%	4.80%
Servicing Assets [Member] | Discount rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	14.80%	16.70%
Servicing Assets [Member] | Discount rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	8.60%	9.00%
Servicing Assets [Member] | Net cumulative expected loss rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	3.70%	2.80%
Servicing Assets [Member] | Net cumulative expected loss rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	36.10%	38.70%
Servicing Assets [Member] | Net cumulative expected loss rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	12.40%	12.50%
Servicing Assets [Member] | Cumulative expected prepayment rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	27.50%	13.90%
Servicing Assets [Member] | Cumulative expected prepayment rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	41.80%	42.90%
Servicing Assets [Member] | Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	32.50%	31.90%
Asset-backed securities related to structured program transaction | Discount rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	3.40%	3.20%
Asset-backed securities related to structured program transaction | Discount rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	20.70%	19.60%
Asset-backed securities related to structured program transaction | Discount rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	8.80%	8.80%
Asset-backed securities related to structured program transaction | Net cumulative expected loss rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	4.50%	6.30%
Asset-backed securities related to structured program transaction | Net cumulative expected loss rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	37.90%	43.90%
Asset-backed securities related to structured program transaction | Net cumulative expected loss rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	19.20%	18.40%
Asset-backed securities related to structured program transaction | Cumulative expected prepayment rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	17.30%	21.00%
Asset-backed securities related to structured program transaction | Cumulative expected prepayment rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	35.10%	33.00%
Asset-backed securities related to structured program transaction | Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	29.40%	30.10%
Loans Invested in by Company | Discount rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	6.00%	5.90%
Loans Invested in by Company | Discount rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	11.50%	16.70%
Loans Invested in by Company | Discount rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	7.80%	9.40%
Loans Invested in by Company | Net cumulative expected loss rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	3.60%	2.60%
Loans Invested in by Company | Net cumulative expected loss rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	36.60%	36.80%
Loans Invested in by Company | Net cumulative expected loss rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	10.90%	13.20%
Loans Invested in by Company | Cumulative expected prepayment rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	27.30%	27.00%
Loans Invested in by Company | Cumulative expected prepayment rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	41.00%	45.50%
Loans Invested in by Company | Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	31.60%	32.50%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Discount rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	6.00%	6.30%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Discount rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	12.00%	16.40%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Discount rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	7.90%	9.10%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Net cumulative expected loss rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	3.60%	2.80%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Net cumulative expected loss rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	34.90%	36.90%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Net cumulative expected loss rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	11.90%	12.80%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Cumulative expected prepayment rates | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	28.70%	27.80%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Cumulative expected prepayment rates | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	38.60%	40.30%
Loans Held for Investment, Notes, Certificates and Secured Borrowings | Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement Input, Percent	31.70%	31.20%