Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Weighted Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair value inputs market servicing rate	0.62%
Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair value inputs market servicing rate	0.62%	0.66%
Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair value inputs market servicing rate	0.62%	0.66%
Level 3 Inputs | Weighted Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair value inputs market servicing rate		0.66%
Discount rates | Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.80%	2.90%
Discount rates | Level 3 Inputs | Minimum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	2.20%	3.40%
Discount rates | Level 3 Inputs | Minimum | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.80%	2.90%
Discount rates | Level 3 Inputs | Minimum | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	8.70%	6.00%
Discount rates | Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.40%	14.80%
Discount rates | Level 3 Inputs | Maximum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	25.10%	20.70%
Discount rates | Level 3 Inputs | Maximum | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.40%	14.80%
Discount rates | Level 3 Inputs | Maximum | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.20%	11.50%
Discount rates | Level 3 Inputs | Weighted Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.90%	8.60%
Discount rates | Level 3 Inputs | Weighted Average | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	8.40%	8.80%
Discount rates | Level 3 Inputs | Weighted Average | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.50%	9.30%
Discount rates | Level 3 Inputs | Weighted Average | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.60%	7.80%
Net cumulative expected loss rates | Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.50%	3.70%
Net cumulative expected loss rates | Level 3 Inputs | Minimum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	5.40%	4.50%
Net cumulative expected loss rates | Level 3 Inputs | Minimum | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.50%	3.70%
Net cumulative expected loss rates | Level 3 Inputs | Minimum | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	5.00%	3.60%
Net cumulative expected loss rates | Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	26.30%	36.10%
Net cumulative expected loss rates | Level 3 Inputs | Maximum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	28.90%	37.90%
Net cumulative expected loss rates | Level 3 Inputs | Maximum | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	26.30%	36.00%
Net cumulative expected loss rates | Level 3 Inputs | Maximum | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	28.00%	36.60%
Net cumulative expected loss rates | Level 3 Inputs | Weighted Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	12.50%	12.40%
Net cumulative expected loss rates | Level 3 Inputs | Weighted Average | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	18.80%	19.20%
Net cumulative expected loss rates | Level 3 Inputs | Weighted Average | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	14.30%	14.40%
Net cumulative expected loss rates | Level 3 Inputs | Weighted Average | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	8.90%	10.90%
Cumulative expected prepayment rates | Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	27.00%	27.50%
Cumulative expected prepayment rates | Level 3 Inputs | Minimum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	6.30%	17.30%
Cumulative expected prepayment rates | Level 3 Inputs | Minimum | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	27.10%	28.50%
Cumulative expected prepayment rates | Level 3 Inputs | Minimum | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	26.80%	27.30%
Cumulative expected prepayment rates | Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	38.90%	41.80%
Cumulative expected prepayment rates | Level 3 Inputs | Maximum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.50%	35.10%
Cumulative expected prepayment rates | Level 3 Inputs | Maximum | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	39.00%	41.70%
Cumulative expected prepayment rates | Level 3 Inputs | Maximum | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	41.00%	41.00%
Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	31.20%	32.50%
Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	24.80%	29.40%
Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average | Loan Trailing Fee Liability
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	31.60%	33.00%
Cumulative expected prepayment rates | Level 3 Inputs | Weighted Average | Loans Invested in by Company
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.50%	31.60%
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	7.60%	6.00%
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	15.00%	12.00%
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	9.40%	7.90%
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	4.30%	3.60%
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	28.10%	34.90%
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	11.20%	11.90%
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	27.30%	28.70%
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	35.70%	38.60%
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	30.40%	31.70%