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Income Taxes - Narrative (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Income Tax Disclosure [Abstract]      
Income Tax Expense (Benefit) $ 5,026,067us-gaap_IncomeTaxExpenseBenefit $ 3,338,715us-gaap_IncomeTaxExpenseBenefit  
Effective tax rate 34.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations 36.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Other Tax Expense (Benefit) 200,000us-gaap_OtherTaxExpenseBenefit    
Valuation allowance 0us-gaap_DeferredTaxAssetsValuationAllowance   0us-gaap_DeferredTaxAssetsValuationAllowance
Excess income tax benefit from stock-based compensation $ 1,343,455us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities $ 166,674us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities