Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 113,863	$ 141,814
Other comprehensive income (loss) before reclassifications	75,143	18,453
Amounts reclassified from accumulated other comprehensive income (loss)	(9,687)	(148)
Other comprehensive income (loss), net of tax	65,456	18,305
Ending Balance	179,319	160,119
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	160,611	190,589
Other comprehensive income (loss) before reclassifications	74,134	17,316
Amounts reclassified from accumulated other comprehensive income (loss)	(9,687)	(148)
Other comprehensive income (loss), net of tax	64,447	17,168
Ending Balance	225,058	207,757
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(46,748)	(48,775)
Other comprehensive income (loss) before reclassifications	1,009	1,137
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Other comprehensive income (loss), net of tax	1,009	1,137
Ending Balance	$ (45,739)	$ (47,638)