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Net Pension Assets - Summary of Components of Net Benefit Expense (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of net defined benefit asset (liability) [line items]      
Current service cost $ (1,767) $ (1,724) $ (1,638)
Interest cost on net benefit obligation 210 173 110
Net benefit expense (1,557) (1,551) (1,528)
Defined benefit obligation [member]      
Disclosure of net defined benefit asset (liability) [line items]      
Current service cost (1,767) (1,724) (1,638)
Interest cost on net benefit obligation (568) (516) (422)
Net benefit expense (2,335) (2,240) (2,060)
Fair value of assets [member]      
Disclosure of net defined benefit asset (liability) [line items]      
Interest cost on net benefit obligation 778 689 532
Net benefit expense $ 778 $ 689 $ 532