Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.01%	0.01%	0.14%
Daily-average amount outstanding	$ 68,073,000	$ 61,551,000	$ 1,059,390,000
Weighted-average interest rate	0.01%	0.05%	2.34%
Federal funds purchased and short term borrowings	$ 47,046,000	$ 59,482,000	$ 62,363,000
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.01%	0.01%	0.14%
Highest amount at a month-end	$ 103,548,000	$ 82,893,000	$ 3,402,566,000
Daily-average amount outstanding	$ 68,073,000	$ 61,551,000	$ 260,322,000
Weighted-average interest rate	0.01%	0.05%	1.86%
Federal Funds Purchased and Repurchase Agreements	$ 47,046,000	$ 59,482,000	$ 62,363,000
Other Short-term Borrowings [Member]
Schedule Of Borrowings [Line Items]
Highest amount at a month-end			5,000,000,000
Daily-average amount outstanding			$ 799,068,000
Weighted-average interest rate			2.49%