Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail) - Level 3 Inputs	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2021	Jun. 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 | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		15.10%	16.40%
Measurement Input, Discount Rate | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		9.90%	9.90%
Measurement Input, Loss Severity | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		2.40%	4.50%
Measurement Input, Loss Severity | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		25.40%	26.30%
Measurement Input, Loss Severity | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		10.30%	12.50%
Measurement Input, Prepayment Rate | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		29.50%	27.00%
Measurement Input, Prepayment Rate | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		46.80%	38.90%
Measurement Input, Prepayment Rate | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		34.70%	31.20%
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		6.00%	2.20%
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		38.00%	25.10%
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		23.20%	8.40%
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		4.70%	5.40%
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		23.80%	28.90%
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		14.30%	18.80%
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		6.80%	6.30%
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		35.20%	30.50%
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		29.20%	24.80%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Discount Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.60%	7.60%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Discount Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		15.30%	15.00%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Discount Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		9.80%	9.40%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Loss Severity | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.30%	3.50%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Loss Severity | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		19.90%	28.10%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Loss Severity | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		9.40%	11.20%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Prepayment Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	27.30%	29.70%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Prepayment Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		38.10%	35.70%
Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Measurement Input, Prepayment Rate | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		33.10%	30.40%
Loans Held For Sale | Measurement Input, Discount Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		5.30%	7.60%
Loans Held For Sale | Measurement Input, Discount Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		11.80%	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		9.50%	8.50%
Loans Held For Sale | Measurement Input, Loss Severity | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		2.40%	5.00%
Loans Held For Sale | Measurement Input, Loss Severity | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		22.60%	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		10.30%	8.20%
Loans Held For Sale | Measurement Input, Prepayment Rate | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		29.80%	27.20%
Loans Held For Sale | Measurement Input, Prepayment Rate | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent		47.00%	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		35.60%	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		10.80%	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		9.30%	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		3.70%	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		13.70%	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		8.10%	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		29.80%	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		38.50%	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		33.50%	30.70%