Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.14%	2.58%	0.92%
Daily-average amount outstanding	$ 1,059,390,000	$ 330,665,000	$ 204,623,000
Weighted-average interest rate	2.34%	1.63%	0.74%
Federal funds purchased and short term borrowings	$ 62,363,000	$ 4,398,378,000	$ 175,099,000
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.14%	1.68%	0.92%
Highest amount at a month-end	$ 3,402,566,000	$ 2,654,416,000	$ 204,977,000
Daily-average amount outstanding	$ 260,322,000	$ 261,200,000	$ 188,459,000
Weighted-average interest rate	1.86%	1.49%	0.69%
Federal Funds Purchased and Repurchase Agreements	$ 62,363,000	$ 198,378,000	$ 175,099,000
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	1,500,000,000
Daily-average amount outstanding	$ 799,068,000	$ 69,465,000	$ 16,164,000
Weighted-average interest rate	2.49%	2.16%	1.27%
Other Short-term Borrowings		$ 4,200,000,000