Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 545	$ 477	$ 850	$ 928
Income before taxes	25,982	19,121	50,429	39,334
Income taxes	(8,504)	(6,887)	(17,181)	(14,211)
Net income	17,478	12,234	33,248	25,123
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(460)	(348)	(274)	(814)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Losses on Defined Benefit Pension Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service (credits) costs	(19)	(19)	(26)	(37)
Transition obligation	0	0	0	0
Actuarial losses (gains)	(16)	(5)	762	(10)
Income before taxes	(35)	(24)	736	(47)
Income taxes	13	9	(280)	17
Net income	(22)	(15)	456	(30)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Realized Gains on Securities Transactions [Member] | Net Unrealized Gains (Losses) on Investment Securities Available-for-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	(545)	(477)	(850)	(928)
Income taxes	207	181	323	352
Net income	(338)	(296)	(527)	(576)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Amortization of Net Unrealized Gains to Income During Period [Member] | Net Unrealized Gains (Losses) on Investment Securities Available-for-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income on investment securities	(162)	(157)	(330)	(328)
Income taxes	62	120	127	120
Net income	$ (100)	$ (37)	$ (203)	$ (208)