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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of investments									$ 450	$ 3,019	$ 1,843
Securities held-to-maturity									31,745	16,615	6,746
Provision for income taxes	(121)us-gaap_IncomeTaxExpenseBenefit	(221)us-gaap_IncomeTaxExpenseBenefit	(231)us-gaap_IncomeTaxExpenseBenefit	(293)us-gaap_IncomeTaxExpenseBenefit	(116)us-gaap_IncomeTaxExpenseBenefit	(308)us-gaap_IncomeTaxExpenseBenefit	(295)us-gaap_IncomeTaxExpenseBenefit	(288)us-gaap_IncomeTaxExpenseBenefit	(866)us-gaap_IncomeTaxExpenseBenefit	(1,007)us-gaap_IncomeTaxExpenseBenefit	(1,392)us-gaap_IncomeTaxExpenseBenefit
Net income	5,623us-gaap_NetIncomeLoss	5,706us-gaap_NetIncomeLoss	5,599us-gaap_NetIncomeLoss	4,932us-gaap_NetIncomeLoss	5,025us-gaap_NetIncomeLoss	5,519us-gaap_NetIncomeLoss	5,026us-gaap_NetIncomeLoss	4,476us-gaap_NetIncomeLoss	21,860us-gaap_NetIncomeLoss	20,046us-gaap_NetIncomeLoss	19,039us-gaap_NetIncomeLoss
Salaries and employee benefits									(35,096)us-gaap_LaborAndRelatedExpense	(35,244)us-gaap_LaborAndRelatedExpense	(32,943)us-gaap_LaborAndRelatedExpense
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									3,241	3,032
Reclassification Out of Accumulated Other Comprehensive Income [Member] \| Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of investments									450	3,019
Provision for income taxes									(171)	(1,179)
Net income									279	1,840
Reclassification Out of Accumulated Other Comprehensive Income [Member] \| Accretion of Unrealized Losses Transferred [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities held-to-maturity									5,192	3,077
Provision for income taxes									(2,004)	(1,191)
Net income									3,188	1,886
Reclassification Out of Accumulated Other Comprehensive Income [Member] \| Prior-Service Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits									(10)	(10)
Reclassification Out of Accumulated Other Comprehensive Income [Member] \| Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits									(367)	(1,144)
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]
Income before taxes									(377)	(1,154)
Provision for income taxes									151	460
Net income									$ (226)	$ (694)