Fair Values of Assets and Liabilities - Valuation Assumption Ranges for MSRs (Detail) - Mortgage Servicing Rights [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected prepayment	10.20%	14.40%
Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected prepayment	3.00%
Option adjusted spread	6.00%
Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected prepayment	13.00%
Option adjusted spread	11.00%
Weighted-average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected prepayment	10.00%
Option adjusted spread	7.00%