Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax at statutory rate, Amount	$ 21,747	$ 19,867	$ 19,814
Income Tax Reconciliation, Bank Owned Life Insurance Income	(1,926)	(1,939)	(2,048)
Tax-exempt interest income, net, Amount	(2,133)	(2,600)	(2,789)
Tax credits, Amount	(134)	(144)	(267)
Other, Amount	126	97	(52)
Total tax provision (benefit), Amount	$ 17,680us-gaap_IncomeTaxExpenseBenefit	$ 15,281us-gaap_IncomeTaxExpenseBenefit	$ 14,658us-gaap_IncomeTaxExpenseBenefit
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Tax at statutory rate, Pretax Income	35.00%	35.00%	35.00%
Income from bank owned life insurance, Pretax Income	(3.00%)	(3.00%)	(4.00%)
Tax-exempt interest income, net, Pretax Income	(4.00%)	(5.00%)	(5.00%)
Tax credits, Pretax Income	0.00%	0.00%	0.00%
Other, Pretax Income	0.00%	0.00%	0.00%
Total tax provision (benefit), Pretax Income	28.00%	27.00%	26.00%

Effective Income Tax Rate Reconciliation Bank Owned Life Insurance

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Income Tax Reconciliation, Bank Owned Life Insurance Income

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

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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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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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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