Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 708	$ 545	$ 1,028	$ 850
Income taxes	(10,850)	(8,504)	(19,440)	(17,181)
Income before taxes	31,420	25,982	58,947	50,429
Net income	20,570	17,478	39,507	33,248
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(574)	(460)	(904)	(274)
Reclassification out of Accumulated Other Comprehensive Income | Net change in defined benefit plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	15	13	27	(280)
Prior service (credits) costs	(19)	(19)	(38)	(26)
Actuarial (gains) losses	(18)	(16)	(34)	762
Income before taxes	(37)	(35)	(72)	736
Net income	(22)	(22)	(45)	456
Reclassification out of Accumulated Other Comprehensive Income | Realized Gains on Securities Transactions | Net change in investment securities available for sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	(708)	(545)	(1,028)	(850)
Income taxes	253	207	367	323
Net income	(455)	(338)	(661)	(527)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of Net Unrealized Gains to Income During Period | Net change in investment securities available for sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	62	62	121	127
Amortization of net unrealized gains to income during the period	(159)	(162)	(319)	(330)
Net income	$ (97)	$ (100)	$ (198)	$ (203)