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Income Taxes - Reconciliation of Differences Between Effective Tax Rate and Federal Statutory Tax Rate (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]      
Federal statutory tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal income tax benefit 3.30%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 3.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 4.30%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Non-deductible employee stock purchase plan expense 0.30%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost 0.30%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost 0.30%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost
Non-deductible meals and entertainment 0.40%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment 0.30%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment 0.30%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment
Other, net 0.10%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther 0.20%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther (0.30%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther
Effective tax rate 39.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations 39.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations 39.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations