Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.92%	0.32%	0.40%
Daily-average amount outstanding	$ 204,623,000	$ 893,822,000	$ 548,005,000
Weighted-average interest rate	0.74%	0.41%	0.31%
Federal funds purchased and short term borrowings	$ 175,099,000	$ 163,442,000	$ 2,132,182,000
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.92%	0.32%	0.06%
Highest amount at a month-end	$ 204,977,000	$ 225,940,000	$ 202,951,000
Daily-average amount outstanding	$ 188,459,000	$ 203,853,000	$ 187,167,000
Weighted-average interest rate	0.69%	0.28%	0.08%
Federal Funds Purchased and Repurchase Agreements	$ 175,099,000	$ 163,442,000	$ 150,546,000
Other Short-term Borrowings [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate			0.43%
Highest amount at a month-end	1,500,000,000	1,974,013,000	$ 1,989,257,000
Daily-average amount outstanding	$ 16,164,000	$ 689,969,000	$ 360,838,000
Weighted-average interest rate	1.27%	0.44%	0.43%
Other Short-term Borrowings			$ 1,981,636,000