Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 320	$ 305
Income taxes	(8,590)	(8,677)
Income before taxes	27,527	24,447
Net income	18,937	15,770
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(330)	186
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	12	(306)
Prior service (credits) costs	(19)	(7)
Actuarial (gains) losses	(16)	791
Income before taxes	(35)	784
Net income	(23)	478
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	(320)	(305)
Income taxes	114	116
Net income	(206)	(189)
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	59	65
Amortization of net unrealized gains to income during the period	(160)	(168)
Net income	$ (101)	$ (103)

The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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