Stockholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	$ (50,074)	$ (35,379)	$ (22,925)
Net change in unrealized (loss) gain	(21,270)	14,821	(105,274)
Reclassification of net (gain) loss realized and included in earnings	3,010	390	8,527
Valuation adjustment for employee benefit plans	(33,971)	(41,244)	82,653
Amortization of unrealized net gain (loss) on securities transferred to held to maturity	3,530	3,297	(6,371)
Income tax expense (benefit)	(18,180)	(8,041)	(8,011)
Accumulated other comprehensive income (loss), Ending balance	(80,595)	(50,074)	(35,379)
Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	18,001	8,263	38,854
Net change in unrealized (loss) gain	(21,581)	15,413	(105,270)
Transfer of net unrealized loss from AFS to HTM, net of cumulative tax effect			36,208
Reclassification of net (gain) loss realized and included in earnings	(165)		(105)
Income tax expense (benefit)	(8,013)	5,675	(38,576)
Accumulated other comprehensive income (loss), Ending balance	4,268	18,001	8,263
Held to Maturity Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	(19,074)	(21,189)	19,090
Transfer of net unrealized loss from AFS to HTM, net of cumulative tax effect			(36,208)
Amortization of unrealized net gain (loss) on securities transferred to held to maturity	3,530	3,297	(6,371)
Income tax expense (benefit)	1,251	1,182	(2,300)
Accumulated other comprehensive income (loss), Ending balance	(16,795)	(19,074)	(21,189)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	(48,626)	(22,453)	(80,688)
Reclassification of net (gain) loss realized and included in earnings	3,175	390	8,331
Valuation adjustment for employee benefit plans	(33,971)	(41,244)	82,653
Income tax expense (benefit)	(11,532)	(14,681)	32,749
Accumulated other comprehensive income (loss), Ending balance	(67,890)	(48,626)	(22,453)
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), Beginning balance	(375)		(181)
Net change in unrealized gain (loss)	311	(592)	(4)
Reclassification of net (gain) loss realized and included in earnings			301
Income tax expense (benefit)	114	(217)	$ 116
Accumulated other comprehensive income (loss), Ending balance	$ (178)	$ (375)