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Property and Equipment, Net - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Line Items]      
Depreciation expense $ 3,635,799us-gaap_Depreciation $ 2,469,922us-gaap_Depreciation $ 1,814,528us-gaap_Depreciation