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Segment information - Additional Information (Detail) - segment
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Segment Reporting Information [Line Items]    
Number of operating segments 1  
Number of reportable segments 1  
Geographic Concentration Risk | Revenue | Outside of United States | Maximum    
Segment Reporting Information [Line Items]    
Revenue 10.00% 10.00%