Change in Accumulated Other Comprehensive Income - Components of Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions									$ 2,369	$ 1,478	$ 1,037
Income before taxes	$ 27,180	$ 19,545	$ 25,982	$ 24,447	$ 21,968	$ 22,504	$ 19,121	$ 20,213	97,154	83,806	72,560
Income tax benefit									(33,074)	(30,273)	(18,803)
Net income	$ 18,110	$ 12,722	$ 17,478	$ 15,770	$ 13,984	$ 14,426	$ 12,234	$ 12,889	64,080	53,533	53,757
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									(1,703)	(1,387)	676
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 costs									(76)	(76)	891
Transition obligation											246
Actuarial losses									348	(20)	991
Income before taxes									272	(96)	2,128
Income tax benefit									(103)	37	(809)
Net income									169	(59)	1,319
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									(2,369)	(1,478)	(1,036)
Income tax benefit									900	562	393
Net income									(1,469)	(916)	$ (643)
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									(651)	(646)
Income tax benefit									248	234
Net income									$ (403)	$ (412)