Income Taxes - Components of Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Included in net income
Current federal	$ 17,378	$ 41,441	$ 14,797
Current state	4,241	1,487	(3,207)
Total current provision	21,619	42,928	11,590
Deferred federal	15,457	21,483	37,403
Deferred state	1,228	2,054	3,517
Total deferred provision	16,685	23,537	40,920
Total included in net income	38,304	66,465	52,510
Included in shareholders' equity
Deferred tax related to retirement benefits	(11,532)	(14,681)	32,749
Deferred tax related to securities	(6,762)	6,857	(40,876)
Deferred tax related to derivatives and hedging	114	(217)	116
Total included in shareholders' equity	$ (18,180)	$ (8,041)	$ (8,011)