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Income Taxes - Summary of reconciliation between federal statutory rate and effective tax rate (Details 1)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Taxes      
Federal statutory rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal benefit 3.30%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 3.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 3.20%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Tax credits (0.70%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits (0.30%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits (0.30%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits
Non-deductible expenses 0.40%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpense 0.50%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpense 0.50%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpense
Other 0.60%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.60%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.40%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective tax rate 38.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations 39.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations 38.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations