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Major Customers Data as Percentage of Total Revenues (Detail) - Revenue - Customer Concentration Risk
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Customer A    
Revenue, Major Customer [Line Items]    
Percentage of total revenues 16.00% 25.00%
Customer B    
Revenue, Major Customer [Line Items]    
Percentage of total revenues 15.00% 17.00%
Customer C    
Revenue, Major Customer [Line Items]    
Percentage of total revenues [1] 11.00%
[1] Less than 10%