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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Directors And Key Management Remuneration [Abstract]      
Emoluments $ 6,686 $ 7,524 $ 9,069
Long-term incentive plans 8,974 4,748 2,923
Total 15,660 12,272 11,992
Pension contributions: defined contribution plans 29 42 80
Gains made on exercise of share options $ 0 $ 0 $ 107