Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Derivative [Line Items]
Notional Amount	$ 27,350,000,000	$ 51,050,000,000
Average Maturity (in years)	9 months 18 days	1 year
Estimated Fair Value Gain (Loss)	$ 1,384,000	$ 1,076,000
Interest Payments On Variable Rate Commercial Real Estate Loans [Member] | Cash Flow Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 25,700,000,000	$ 49,400,000,000
Average Maturity (in years)	8 months 12 days	10 months 24 days
Weighted-Average Rate, Fixed	1.28%	2.22%
Weighted-Average Rate, Variable	0.08%	0.15%
Estimated Fair Value Gain (Loss)	$ 656,000	$ 425,000
Fixed Rate Long-Term Borrowings [Member] | Fair Value Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 1,650,000,000	$ 1,650,000,000
Average Maturity (in years)	2 years 7 months 6 days	3 years 3 months 18 days
Weighted-Average Rate, Fixed	2.86%	2.86%
Weighted-Average Rate, Variable	0.73%	0.79%
Estimated Fair Value Gain (Loss)	$ 728,000	$ 651,000