Securities - Sales of Securities Available for Sale (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 746,524	$ 7,980,049	$ 8,993,963	$ 9,040,245
Gross realized gains	0	1	0	11,134
Gross realized losses	(4,867)	(38)	(4,917)	(38)
Tax (expense)benefit of securities gains/losses	$ 1,703	$ 13	$ 1,721	$ (3,884)