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Segment Information and Geographic Data - Additional Information (Detail) - Segment
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Segment Reporting Information [Line Items]    
Number of operating segment 1  
Revenue [Member] | Outside Of United States [Member] | Geographic Concentration Risk [Member]    
Segment Reporting Information [Line Items]    
Concentration risk, percentage 36.00% 30.00%