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Geographic Information and Concentrations of Risk - Schedule of Geographic Concentration of Net Revenues (Detail) - Net Revenues - Geographic Concentration
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]      
Concentration percentage 100.00% 100.00% 100.00%
United States and Canada      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Concentration percentage 71.40% 72.90% 89.10%
South Africa      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Concentration percentage 17.90% 16.60% 4.60%
Other      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Concentration percentage 10.70% 10.50% 6.30%