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Total Revenue of Major Clients (Parenthetical) (Detail) - Total Revenue
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Revenue, Major Customer [Line Items]        
Concentration risk, percentage 100.00% 100.00% 100.00% 100.00%
Credit Concentration Risk | Client A        
Revenue, Major Customer [Line Items]        
Concentration risk, percentage 0.00% [1] 11.00% 0.00% [1] 0.00%
Credit Concentration Risk | Client A | Maximum        
Revenue, Major Customer [Line Items]        
Concentration risk, percentage     10.00%  
[1] Client accounted for less than 10% of total revenue.