Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.32%	0.40%	0.07%
Daily-average amount outstanding	$ 893,822,000	$ 548,005,000	$ 214,736,000
Weighted-average interest rate	0.41%	0.31%	0.05%
Federal Funds Purchased and Repurchase Agreements	$ 163,442,000	$ 150,546,000
Other Short-term Borrowings		1,981,636,000
Federal funds purchased and short term borrowings	$ 163,442,000	$ 2,132,182,000	$ 192,676,000
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate	0.32%	0.06%	0.07%
Highest amount at a month-end	$ 225,940,000	$ 202,951,000	$ 280,350,000
Daily-average amount outstanding	$ 203,853,000	$ 187,167,000	$ 214,736,000
Weighted-average interest rate	0.28%	0.08%	0.05%
Federal Funds Purchased and Repurchase Agreements	$ 163,442,000	$ 150,546,000	$ 192,676,000
Other Short-term Borrowings [Member]
Schedule Of Borrowings [Line Items]
Weighted-average interest rate		0.43%
Highest amount at a month-end	1,974,013,000	$ 1,989,257,000
Daily-average amount outstanding	$ 689,969,000	$ 360,838,000
Weighted-average interest rate	0.44%	0.43%
Other Short-term Borrowings		$ 1,981,636,000