Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	2.58%	0.92%	0.32%
Daily-average amount outstanding	$ 330,665,000	$ 204,623,000	$ 893,822,000
Weighted-average interest rate	1.63%	0.74%	0.41%
Federal funds purchased and short term borrowings	$ 4,398,378,000	$ 175,099,000	$ 163,442,000
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	1.68%	0.92%	0.32%
Highest amount at a month-end	$ 2,654,416,000	$ 204,977,000	$ 225,940,000
Daily-average amount outstanding	$ 261,200,000	$ 188,459,000	$ 203,853,000
Weighted-average interest rate	1.49%	0.69%	0.28%
Federal Funds Purchased and Repurchase Agreements	$ 198,378,000	$ 175,099,000	$ 163,442,000
Other Short-term Borrowings [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	2.63%
Highest amount at a month-end	$ 4,200,000,000	1,500,000,000	1,974,013,000
Daily-average amount outstanding	$ 69,465,000	$ 16,164,000	$ 689,969,000
Weighted-average interest rate	2.16%	1.27%	0.44%
Other Short-term Borrowings	$ 4,200,000,000