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Net Pension Assets - Summary of Additional Information about Sensitivity Analysis for Actuarial Assumptions (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Discount rate (0.5% movement) [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Increase $ (506) $ (410) $ (366)
Decrease 537 434 388
Compensation - salary increase [member]      
Disclosure of sensitivity analysis for actuarial assumptions [line items]      
Increase 99 122 114
Decrease $ (89) $ (117) $ (109)