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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 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 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 74.40% 73.80% 72.80% 75.40%
South Africa        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Concentration percentage 16.90% 15.00% 17.30% 14.30%
Other        
Revenues from External Customers and Long-Lived Assets [Line Items]        
Concentration percentage 8.70% 11.20% 9.90% 10.30%