Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ (24,623)	$ 113,863
Other comprehensive income (loss) before reclassifications			76,455	111,191
Amounts reclassified from accumulated other comprehensive income (loss)			5,843	(9,541)
Other comprehensive income (loss), net of tax	$ 6,365	$ (65,844)	82,298	101,650
Ending Balance	57,675	215,513	57,675	215,513
Securities Available For Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			16,153	160,611
Other comprehensive income (loss) before reclassifications			76,455	108,702
Amounts reclassified from accumulated other comprehensive income (loss)			3,196	(9,662)
Other comprehensive income (loss), net of tax			79,651	99,040
Ending Balance	95,804	259,651	95,804	259,651
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			(40,776)	(46,748)
Other comprehensive income (loss) before reclassifications			0	2,489
Amounts reclassified from accumulated other comprehensive income (loss)			2,647	121
Other comprehensive income (loss), net of tax			2,647	2,610
Ending Balance	$ (38,129)	$ (44,138)	$ (38,129)	$ (44,138)