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Geographic Information and Concentrations of Risk - Schedule of Geographic Concentration of Net Revenues (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]                    
Net revenues $ 56,043 $ 50,630 $ 49,057 $ 46,733 $ 46,534 $ 57,461 $ 59,913 $ 55,389 $ 202,463 $ 219,297
United States and Canada                    
Revenues from External Customers and Long-Lived Assets [Line Items]                    
Net revenues                 139,246 156,661
South Africa                    
Revenues from External Customers and Long-Lived Assets [Line Items]                    
Net revenues                 38,608 39,182
Other                    
Revenues from External Customers and Long-Lived Assets [Line Items]                    
Net revenues                 $ 24,609 $ 23,454