Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Borrowings [Line Items]
Federal Funds Purchased and Repurchase Agreements	$ 150,546	$ 192,676
Other Short-term borrowings	1,981,636
Federal funds purchased and short term borrowings	$ 2,132,182	$ 192,676	$ 260,455
Weighted-average interest rate	0.40%	0.07%	0.04%
Daily-average amount outstanding	$ 548,005	$ 214,736	$ 390,034
Weighted-average interest rate	0.31%	0.05%	0.11%
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Federal Funds Purchased and Repurchase Agreements	$ 150,546	$ 192,676	$ 260,455
Weighted-average interest rate	0.06%	0.07%	0.04%
Highest amount at a month-end	$ 202,951	$ 280,350	$ 563,879
Daily-average amount outstanding	$ 187,167	$ 214,736	$ 390,034
Weighted-average interest rate	0.08%	0.05%	0.11%
Other Short-term Borrowings [Member]
Schedule Of Borrowings [Line Items]
Other Short-term borrowings	$ 1,981,636
Weighted-average interest rate	0.43%
Highest amount at a month-end	$ 1,989,257
Daily-average amount outstanding	$ 360,838
Weighted-average interest rate	0.43%