Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 141,814	$ 140,434
Other comprehensive income (loss) before reclassifications			(59,369)	32,184
Amounts reclassified from accumulated other comprehensive income (loss)			(148)	(12,150)
Other comprehensive income (loss), net of tax	$ (77,822)	$ 17,720	(59,517)	20,034
Ending Balance	82,297	160,468	82,297	160,468
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			190,589	146,672
Other comprehensive income (loss) before reclassifications			(61,643)	31,311
Amounts reclassified from accumulated other comprehensive income (loss)			(148)	(2)
Other comprehensive income (loss), net of tax			(61,791)	31,309
Ending Balance	128,798	177,981	128,798	177,981
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(48,775)	(26,131)
Other comprehensive income (loss) before reclassifications			2,274	873
Amounts reclassified from accumulated other comprehensive income (loss)			0	0
Other comprehensive income (loss), net of tax			2,274	873
Ending Balance	(46,501)	(25,258)	(46,501)	(25,258)
Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			0	19,893
Other comprehensive income (loss) before reclassifications			0	0
Amounts reclassified from accumulated other comprehensive income (loss)			0	(12,148)
Other comprehensive income (loss), net of tax			0	(12,148)
Ending Balance	$ 0	$ 7,745	$ 0	$ 7,745