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, 2021	Dec. 31, 2020
Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.62%	0.62%
Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.62%	0.62%
Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.62%	0.62%
Measurement Input, Discount Rate | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.50%	4.80%
Measurement Input, Discount Rate | Minimum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	6.00%	2.20%
Measurement Input, Discount Rate | Minimum | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.90%	7.60%
Measurement Input, Discount Rate | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	15.10%	16.40%
Measurement Input, Discount Rate | Maximum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	38.00%	25.10%
Measurement Input, Discount Rate | Maximum | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	15.80%	15.00%
Measurement Input, Discount Rate | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.00%	9.90%
Measurement Input, Discount Rate | Weighted- Average | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	25.00%	8.40%
Measurement Input, Discount Rate | Weighted- Average | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.00%	9.40%
Measurement Input, Loss Severity | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	2.50%	4.50%
Measurement Input, Loss Severity | Minimum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	3.30%	5.40%
Measurement Input, Loss Severity | Minimum | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	3.00%	4.30%
Measurement Input, Loss Severity | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	25.50%	26.30%
Measurement Input, Loss Severity | Maximum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	19.20%	28.90%
Measurement Input, Loss Severity | Maximum | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	18.70%	28.10%
Measurement Input, Loss Severity | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.00%	12.50%
Measurement Input, Loss Severity | Weighted- Average | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	12.00%	18.80%
Measurement Input, Loss Severity | Weighted- Average | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	8.70%	11.20%
Measurement Input, Prepayment Rate | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	32.30%	27.00%
Measurement Input, Prepayment Rate | Minimum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	15.40%	6.30%
Measurement Input, Prepayment Rate | Minimum | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	32.00%	27.30%
Measurement Input, Prepayment Rate | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	45.50%	38.90%
Measurement Input, Prepayment Rate | Maximum | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	35.10%	30.50%
Measurement Input, Prepayment Rate | Maximum | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	41.00%	35.70%
Measurement Input, Prepayment Rate | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	37.10%	31.20%
Measurement Input, Prepayment Rate | Weighted- Average | Asset-backed securities related to Structured Program transactions
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	28.90%	24.80%
Measurement Input, Prepayment Rate | Weighted- Average | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	37.50%	30.40%
Loans Held For Sale | Measurement Input, Discount Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	5.20%	7.60%
Loans Held For Sale | Measurement Input, Discount Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	11.60%	16.00%
Loans Held For Sale | Measurement Input, Discount Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.10%	8.50%
Loans Held For Sale | Measurement Input, Loss Severity | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	2.50%	5.00%
Loans Held For Sale | Measurement Input, Loss Severity | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	25.60%	28.00%
Loans Held For Sale | Measurement Input, Loss Severity | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.80%	8.20%
Loans Held For Sale | Measurement Input, Prepayment Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.60%	27.20%
Loans Held For Sale | Measurement Input, Prepayment Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	44.80%	41.20%
Loans Held For Sale | Measurement Input, Prepayment Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	36.00%	30.40%
Loans Invested in by Company | Measurement Input, Discount Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.70%	7.50%
Loans Invested in by Company | Measurement Input, Discount Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	11.00%	16.10%
Loans Invested in by Company | Measurement Input, Discount Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.40%	8.80%
Loans Invested in by Company | Measurement Input, Loss Severity | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	2.90%	5.00%
Loans Invested in by Company | Measurement Input, Loss Severity | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	12.30%	26.30%
Loans Invested in by Company | Measurement Input, Loss Severity | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.30%	10.30%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.60%	26.80%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	40.20%	39.70%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	35.20%	30.70%