Income Taxes - Schedule of Deferred tax assets and liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Allowance for loan losses	$ 16,386	$ 17,414	$ 19,901
Compensation and benefits	8,764	2,324	10,192
Security gains and losses	2,817	3,261	2,261
Purchase accounting adjustments	1,497	3,544	8,037
Non-accrual interest income	2,129	1,850	1,745
Tax credit carryforwards	10,163	11,517	11,838
Federal net operating loss carryforwards	597	1,415	2,316
Fair value adjustments on securities available-for-sale		2,772
Other	3,327us-gaap_DeferredTaxAssetsOther	3,175us-gaap_DeferredTaxAssetsOther	3,070us-gaap_DeferredTaxAssetsOther
Gross deferred tax assets	45,680us-gaap_DeferredTaxAssetsGross	47,272us-gaap_DeferredTaxAssetsGross	59,360us-gaap_DeferredTaxAssetsGross
Deferred tax liabilities:
Depreciation and amortization	(1,900)	(1,416)	(962)
Accretion on securities	(295)	(262)	(397)
Fair value adjustments on securities available-for-sale	(2,297)		(8,806)
Other	(1,728)us-gaap_DeferredTaxLiabilitiesOther	(983)us-gaap_DeferredTaxLiabilitiesOther	(1,257)us-gaap_DeferredTaxLiabilitiesOther
Gross deferred tax liabilities	(6,220)us-gaap_DeferredIncomeTaxLiabilities	(2,661)us-gaap_DeferredIncomeTaxLiabilities	(11,422)us-gaap_DeferredIncomeTaxLiabilities
Net deferred tax assets	$ 39,460us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 44,611us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 47,938us-gaap_DeferredTaxAssetsLiabilitiesNet

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Amount of deferred tax liability attributable to taxable temporary differences.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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The tax effect as of the balance sheet date of the amount of future tax deductions arising from all unused tax credit carryforwards which have been reduced by a valuation allowance.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from share-based compensation.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the provision for loan losses.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from unrealized losses on available-for-sale securities.

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Amount of deferred tax liability attributable to taxable temporary differences not separately disclosed.

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Deferred tax assets non accrual interest income.

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Deferred tax assets purchase accounting adjustments.

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Deferred tax assets securities gains and losses.

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Deferred tax liabilities accretion on securities.

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Deferred tax liabilities unrealized gains (losses) on available for sale securities.

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