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Employee Benefit Plans - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]      
Contribution for multi-employer defined contribution plan $ 4,923,483us-gaap_DefinedBenefitPlanContributionsByEmployer $ 2,958,400us-gaap_DefinedBenefitPlanContributionsByEmployer $ 2,277,079us-gaap_DefinedBenefitPlanContributionsByEmployer