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Net Pension Assets - Summary of Reconciliation of Net Pension Asset (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]      
Beginning balance $ 8,826 $ 6,050 $ 2,522
Current service cost (1,767) (1,724) (1,638)
Interest (cost) income 210 173 110
Return on plan assets greater (less) than discount rate (21) 518 701
Experience (gain) loss (2,033) (1,052) (809)
Investment return 88 27 105
Gross benefits paid (440) (513) (599)
Assumption changes (226) (67) 222
Employer contributions (1,677) 3,970 3,749
Benefits paid 215 236 183
Adjustments 10 1,208 1,504
Ending balance 3,185 8,826 6,050
Defined benefit obligation [member]      
Disclosure of net defined benefit asset (liability) [line items]      
Beginning balance (16,498) (14,468) (12,778)
Current service cost (1,767) (1,724) (1,638)
Interest (cost) income (568) (516) (422)
Experience (gain) loss (2,033) (1,052) (809)
Assumption changes (226) (67) 222
Benefits paid 1,095 1,329 957
Ending balance (19,997) (16,498) (14,468)
Fair value of assets [member]      
Disclosure of net defined benefit asset (liability) [line items]      
Beginning balance 25,946 22,273 18,559
Interest (cost) income 778 689 532
Return on plan assets greater (less) than discount rate (21) 518 701
Investment return 88 27 105
Gross benefits paid (440) (513) (599)
Employer contributions (1,677) 3,970 3,749
Benefits paid (880) (1,018) (774)
Ending balance 23,794 25,946 22,273
Other employee benefits liability [member]      
Disclosure of net defined benefit asset (liability) [line items]      
Beginning balance (622) (1,755) (3,259)
Benefits paid   (75)  
Adjustments 10 1,208 1,504
Ending balance $ (612) $ (622) $ (1,755)