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Net Pension Assets - Summary of Expected Contribution Payments to Defined Benefit Plan (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of defined benefit plans [line items]    
Total expected payments $ 25,619 $ 21,471
Up to one year [member]    
Disclosure of defined benefit plans [line items]    
Total expected payments 3,424 2,823
More than one year but less than five years [member]    
Disclosure of defined benefit plans [line items]    
Total expected payments 10,794 9,195
Over five years [member]    
Disclosure of defined benefit plans [line items]    
Total expected payments $ 11,401 $ 9,453