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Segment Reporting - Consolidated Sales Revenues by Similar Products and Services (Detail) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Segment Reporting, Revenue Reconciling Item [Line Items]        
Net revenues $ 112,183,360 $ 88,844,059 $ 214,909,107 $ 172,580,065
Marketplace [Member]
       
Segment Reporting, Revenue Reconciling Item [Line Items]        
Net revenues 77,915,508 61,936,103 149,630,271 122,218,201
Non-marketplace [Member]
       
Segment Reporting, Revenue Reconciling Item [Line Items]        
Net revenues $ 34,267,852 $ 26,907,956 $ 65,278,836 $ 50,361,864