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Effective Tax Rate Differs from Statutory United States Federal Income Tax Rate (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]      
Statutory Rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Increases (Decreases) in United States Federal Taxes      
State Income Taxes, Net of Federal Income Tax Benefit 2.60%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 3.10%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 2.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Federal Tax Credits (1.80%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther (1.70%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther
Other, Net (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (1.60%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.10%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective Tax Rate 35.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations 34.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations 37.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations