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Geographic Information and Concentrations of Risk - Schedule of Geographic Concentration of Net Revenues (Detail) - Net Revenues - Geographic Concentration
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]        
Concentration percentage 100.00% 100.00% 100.00% 100.00%
United States and Canada        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Concentration percentage 73.20% 72.40% 72.90% 74.60%
South Africa        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Concentration percentage 17.00% 17.70% 17.20% 15.30%
Other        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Concentration percentage 9.80% 9.90% 9.90% 10.10%