Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.01%	0.14%	2.58%
Daily-average amount outstanding	$ 61,551,000	$ 1,059,390,000	$ 330,665,000
Weighted-average interest rate	0.05%	2.34%	1.63%
Federal funds purchased and short term borrowings	$ 59,482,000	$ 62,363,000	$ 4,398,378,000
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.01%	0.14%	1.68%
Highest amount at a month-end	$ 82,893,000	$ 3,402,566,000	$ 2,654,416,000
Daily-average amount outstanding	$ 61,551,000	$ 260,322,000	$ 261,200,000
Weighted-average interest rate	0.05%	1.86%	1.49%
Federal Funds Purchased and Repurchase Agreements	$ 59,482,000	$ 62,363,000	$ 198,378,000
Other Short-term Borrowings [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate			2.63%
Highest amount at a month-end		5,000,000,000	$ 4,200,000,000
Daily-average amount outstanding		$ 799,068,000	$ 69,465,000
Weighted-average interest rate		2.49%	2.16%
Other Short-term Borrowings			$ 4,200,000,000