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Segment reporting - Allocation of Long-Lived Tangible Assets Based on Geography (Detail) (USD $)
Jun. 30, 2013
Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries long-lived tangible assets $ 70,562,482 $ 30,944,145
Total long-lived tangible assets 80,081,659 37,726,222
United States [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries long-lived tangible assets 9,519,177 6,782,077
Argentina [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries long-lived tangible assets 33,568,284 16,955,438
Brazil [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries long-lived tangible assets 3,459,910 2,421,618
Mexico [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries long-lived tangible assets 572,537 378,653
Venezuela [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries long-lived tangible assets 29,755,221 8,455,816
Other Countries [Member]
    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Other countries long-lived tangible assets $ 3,206,530 $ 2,732,620