Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 113,863	$ 141,814
Other comprehensive income (loss) before reclassifications			111,191	(7,784)
Amounts reclassified from accumulated other comprehensive income (loss)			(9,541)	(114)
Other comprehensive income (loss), net of tax	$ (65,844)	$ 51,619	101,650	(7,898)
Ending Balance	215,513	133,916	215,513	133,916
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			160,611	190,589
Other comprehensive income (loss) before reclassifications			108,702	(11,194)
Amounts reclassified from accumulated other comprehensive income (loss)			(9,662)	(114)
Other comprehensive income (loss), net of tax			99,040	(11,308)
Ending Balance	259,651	179,281	259,651	179,281
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(46,748)	(48,775)
Other comprehensive income (loss) before reclassifications			2,489	3,410
Amounts reclassified from accumulated other comprehensive income (loss)			121	0
Other comprehensive income (loss), net of tax			2,610	3,410
Ending Balance	$ (44,138)	$ (45,365)	$ (44,138)	$ (45,365)