Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 736	$ 1,040	$ 1,764	$ 1,890
Income taxes	(10,942)	(6,823)	(30,382)	(24,004)
Income before taxes	31,511	19,545	90,458	69,974
Net income	20,569	12,722	60,076	45,970
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(596)	(767)	(1,500)	(1,041)
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	14	42	(266)
Prior service (credits) costs	(19)	(18)	(57)	(44)
Actuarial (gains) losses	(18)	(16)	(52)	746
Income before taxes	(37)	(34)	(109)	702
Net income	(22)	(20)	(67)	436
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	(736)	(1,040)	(1,764)	(1,890)
Income taxes	261	395	628	718
Net income	(475)	(645)	(1,136)	(1,172)
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	60	60	181	187
Amortization of net unrealized gains to income during the period	(159)	(162)	(478)	(492)
Net income	$ (99)	$ (102)	$ (297)	$ (305)