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Percentage of Revenue by Geographic Area (Detail) - Geographic Concentration Risk - Total Revenue
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Concentration Risk [Line Items]        
Revenues, percentage by country 100.00% 100.00% 100.00% 100.00%
United States        
Concentration Risk [Line Items]        
Revenues, percentage by country 45.00% 45.00% 40.00% 40.00%
Sweden        
Concentration Risk [Line Items]        
Revenues, percentage by country 15.00% 23.00% 23.00% 29.00%
United Kingdom        
Concentration Risk [Line Items]        
Revenues, percentage by country 10.00% 11.00% 13.00% 14.00%
Other        
Concentration Risk [Line Items]        
Revenues, percentage by country 30.00% 21.00% 24.00% 17.00%