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Directors' and key management remuneration - Summary of aggregate remuneration calculated in accordance with UK companies act 2006 (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Directors And Key Management Remuneration [Abstract]      
Emoluments $ 9,069 $ 8,339 $ 9,186
Long term incentive plans 2,923 4,685 3,071
Total 11,992 13,024 12,257
Pension contributions: defined contribution plans 80 $ 135 $ 69
Gains made on exercise of share options $ 107