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Major Customers Data as Percentage of Total Revenues (Detail) - Revenue - Customer Concentration Risk
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Customer A      
Revenue, Major Customer [Line Items]      
Percentage of total revenues 23.00% 27.00% 31.00%
Customer B      
Revenue, Major Customer [Line Items]      
Percentage of total revenues 17.00% 19.00% 0.00% [1]
[1] Less than 10%