Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 113,863	$ 141,814
Other comprehensive income (loss) before reclassifications			177,060	(59,369)
Amounts reclassified from accumulated other comprehensive income (loss)			(9,566)	(148)
Other comprehensive income (loss), net of tax	$ 102,038	$ (77,822)	167,494	(59,517)
Ending Balance	281,357	82,297	281,357	82,297
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			160,611	190,589
Other comprehensive income (loss) before reclassifications			175,601	(61,643)
Amounts reclassified from accumulated other comprehensive income (loss)			(9,687)	(148)
Other comprehensive income (loss), net of tax			165,914	(61,791)
Ending Balance	326,525	128,798	326,525	128,798
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(46,748)	(48,775)
Other comprehensive income (loss) before reclassifications			1,459	2,274
Amounts reclassified from accumulated other comprehensive income (loss)			121	0
Other comprehensive income (loss), net of tax			1,580	2,274
Ending Balance	$ (45,168)	$ (46,501)	$ (45,168)	$ (46,501)