Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail)	6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
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)	0.66%
Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.66%		0.66%
Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Total market servicing rates (percent per annum on outstanding principal balance)	0.66%		0.66%
Level 3 Inputs | 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)			0.66%
Measurement Input, Discount Rate | Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.80%		2.90%
Measurement Input, Discount Rate | Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.40%		14.80%
Measurement Input, Discount Rate | Level 3 Inputs | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.00%		8.60%
Measurement Input, Discount Rate | Loan Trailing Fee Liability | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.80%	2.90%
Measurement Input, Discount Rate | Loan Trailing Fee Liability | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.40%	14.80%
Measurement Input, Discount Rate | Loan Trailing Fee Liability | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.70%	9.30%
Measurement Input, Discount Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.60%		6.00%
Measurement Input, Discount Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.00%		12.00%
Measurement Input, Discount Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.60%		7.90%
Measurement Input, Loss Severity | Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	5.50%		3.70%
Measurement Input, Loss Severity | Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	31.80%		36.10%
Measurement Input, Loss Severity | Level 3 Inputs | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	14.70%		12.40%
Measurement Input, Loss Severity | Loan Trailing Fee Liability | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	5.60%	3.70%
Measurement Input, Loss Severity | Loan Trailing Fee Liability | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	31.80%	36.00%
Measurement Input, Loss Severity | Loan Trailing Fee Liability | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.90%	14.40%
Measurement Input, Loss Severity | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	5.10%		3.60%
Measurement Input, Loss Severity | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	29.00%		34.90%
Measurement Input, Loss Severity | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	13.30%		11.90%
Measurement Input, Prepayment Rate | Level 3 Inputs | Minimum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	25.70%		27.50%
Measurement Input, Prepayment Rate | Level 3 Inputs | Maximum | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	37.40%		41.80%
Measurement Input, Prepayment Rate | Level 3 Inputs | Weighted- Average | Servicing Assets
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.30%		32.50%
Measurement Input, Prepayment Rate | Loan Trailing Fee Liability | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	25.80%	28.50%
Measurement Input, Prepayment Rate | Loan Trailing Fee Liability | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	37.50%	41.70%
Measurement Input, Prepayment Rate | Loan Trailing Fee Liability | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.60%	33.00%
Measurement Input, Prepayment Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	26.30%		28.70%
Measurement Input, Prepayment Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	38.20%		38.60%
Measurement Input, Prepayment Rate | Loans Held for Investment, Loans Held for Sale, Notes, Certificates and Secured Borrowings | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.00%		31.70%
Loans Invested in by Company | Measurement Input, Discount Rate | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	4.80%		6.00%
Loans Invested in by Company | Measurement Input, Discount Rate | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.10%		11.50%
Loans Invested in by Company | Measurement Input, Discount Rate | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.60%		7.80%
Loans Invested in by Company | Measurement Input, Loss Severity | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	6.20%		3.60%
Loans Invested in by Company | Measurement Input, Loss Severity | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	31.90%		36.60%
Loans Invested in by Company | Measurement Input, Loss Severity | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	14.10%		10.90%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	26.20%		27.30%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	37.80%		41.00%
Loans Invested in by Company | Measurement Input, Prepayment Rate | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.70%		31.60%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Discount Rate | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	1.10%		3.40%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Discount Rate | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	20.00%		20.70%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Discount Rate | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	11.30%		8.80%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Loss Severity | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	5.70%		4.50%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Loss Severity | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	35.80%		37.90%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Loss Severity | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	20.90%		19.20%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Prepayment Rate | Level 3 Inputs | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	9.40%		17.30%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Prepayment Rate | Level 3 Inputs | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	30.40%		35.10%
Asset-backed Securities Related to Structured Program Transactions | Measurement Input, Prepayment Rate | Level 3 Inputs | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	23.90%		29.40%