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Defined benefit plans - Summary of Amounts Recognised in the Income Statement (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost $ 3,204 $ 2,979
Interest cost 2,005 2,128
Past Service Cost 146 414
Expected return on plan assets (1,800) (2,070)
Defined benefit plans expense 3,555 3,451
Defined Benefit Pension Plans [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost 2,935 2,102
Interest cost 601 620
Past Service Cost 0 206
Expected return on plan assets (73) (87)
Defined benefit plans expense 3,463 2,841
Supplemental pension plan [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Current service cost 269 877
Interest cost 1,404 1,508
Past Service Cost 146 208
Expected return on plan assets (1,727) (1,983)
Defined benefit plans expense $ 92 $ 610