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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Directors And Key Management Remuneration [Abstract]      
Emoluments $ 8,339 $ 9,186 $ 10,590
Long-term incentive plans 4,685 3,071 2,118
Total 13,024 12,257 12,708
Pension contributions: defined contribution plans $ 135 $ 69 52
Gains made on exercise of share options     $ 1