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Defined Benefit Plans - Summary of Amounts Recognised in the Income Statement (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost $ 2,979 $ 2,003
Interest cost 2,128 1,816
Past Service Cost 414 3,687
Expected return on plan assets (2,070) (1,904)
Defined benefit plans expense 3,451 5,602
Defined Benefit Pension Plans [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost 2,102 520
Interest cost 620 476
Past Service Cost 206 3,494
Expected return on plan assets (87) (81)
Defined benefit plans expense 2,841 4,409
Supplemental pension plan [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost 877 1,483
Interest cost 1,508 1,340
Past Service Cost 208 193
Expected return on plan assets (1,983) (1,823)
Defined benefit plans expense $ 610 $ 1,193