Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 1,040	$ 76	$ 1,890	$ 1,004
Income before taxes	19,545	22,504	69,974	61,838
Income taxes	(6,823)	(8,078)	(24,004)	(22,289)
Net income	12,722	14,426	45,970	39,549
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(767)	(166)	(1,041)	(980)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Change in Defined Benefit Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service (credits) costs	(18)	(19)	(44)	(57)
Transition obligation	0	0	0	0
Actuarial losses (gains)	(16)	(5)	746	(15)
Income before taxes	(34)	(24)	702	(72)
Income taxes	14	9	(266)	27
Net income	(20)	(15)	436	(45)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Realized Gains on Securities Transactions [Member] | Net Change in Investment Securities Available-For-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	(1,040)	(76)	(1,890)	(1,004)
Income taxes	395	29	718	381
Net income	(645)	(47)	(1,172)	(623)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Amortization of Net Unrealized Gains to Income During Period [Member] | Net Change in Investment Securities Available-For-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income on investment securities	(162)	(159)	(492)	(487)
Income taxes	60	55	187	175
Net income	$ (102)	$ (104)	$ (305)	$ (312)