Income Taxes - Components of Net Deferred Tax Liability (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Allowance for loan losses	$ 5,076	$ 5,084
Unrealized loss on securities available for sale	0	4,085
Fair value adjustments - business combination	2,466	3,541
Deferred compensation	2,238	2,179
Impaired security valuation	479	590
Net operating loss carryover	1,646	2,155
Post-retirement benefits	1,423	1,308
Unrealized loss on interest rate swap	331	391
Nonaccrual loan interest	598	516
Accrued expenses	974	817
Other	391us-gaap_DeferredTaxAssetsOther	296us-gaap_DeferredTaxAssetsOther
Deferred tax assets	15,622us-gaap_DeferredTaxAssetsNet	20,962us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities:
Unrealized gain on securities available for sale	1,499	0
Premises and equipment	1,528	1,527
Intangibles - section 197	4,900	5,130
Mortgage servicing rights	299	317
Other	124us-gaap_DeferredTaxLiabilitiesOther	328us-gaap_DeferredTaxLiabilitiesOther
Deferred tax liabilities	8,350us-gaap_DeferredTaxLiabilities	7,302us-gaap_DeferredTaxLiabilities
Net deferred tax asset	$ 7,272us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 13,660us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred tax assets fair value adjustments.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from derivative instruments.

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