Change in Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 76	$ 36	$ 1,004	$ 979
Interest income on investment securities	875	837	2,587	2,443
Income before taxes	22,504	17,555	61,838	53,274
Income taxes	(8,078)	(6,142)	(22,289)	(12,225)
Net income	14,426	11,413	39,549	41,049
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(166)	(18)	(980)	(543)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Amortization of Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service (credits) costs	(19)	22	(57)	905
Transition obligation				245
Actuarial (gains) losses	(5)	(14)	(15)	(1,046)
Income before taxes	(24)	8	(72)	104
Income taxes	9	(3)	27	(39)
Net income	(15)	5	(45)	65
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	(76)	(36)	(1,004)	(979)
Income taxes	29	13	381	371
Net income	(47)	$ (23)	(623)	$ (608)
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	(159)		(487)
Income taxes	55		175
Net income	$ (104)		$ (312)