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Segment reporting - Allocation of Goodwill and Intangible Assets Based on Geography (Detail) (USD $)
Jun. 30, 2013
Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries goodwill and intangible assets $ 66,260,035 $ 67,624,923
Total goodwill and intangible assets 66,275,789 67,645,928
United States [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
US intangible assets 15,754 21,005
Argentina [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries goodwill and intangible assets 19,067,487 20,328,154
Brazil [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries goodwill and intangible assets 11,129,343 10,724,007
Mexico [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries goodwill and intangible assets 14,667,740 14,644,795
Venezuela [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries goodwill and intangible assets 6,980,576 6,595,117
Other Countries [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries goodwill and intangible assets $ 14,414,889 $ 15,332,850