Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail) - Level 3 Inputs - Servicing Assets	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Minimum
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
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
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%
Discount rates | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	7.50%	4.80%
Discount rates | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	16.40%	16.40%
Discount rates | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.00%	9.90%
Net cumulative expected loss rates | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	2.40%	4.50%
Net cumulative expected loss rates | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	26.40%	26.30%
Net cumulative expected loss rates | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	10.20%	12.50%
Cumulative expected prepayment rates | Minimum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	32.10%	27.00%
Cumulative expected prepayment rates | Maximum
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	45.90%	38.90%
Cumulative expected prepayment rates | Weighted- Average
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Measurement input, percent	38.40%	31.20%