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Defined benefit plans - Summary of Movement in the Fair Value of Plan Assets of the Year (Detail) - Plan assets [member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of fair value of plan assets [line items]    
Beginning Balance $ 56,304 $ 51,360
Interest income on plan assets 1,799 2,069
Actuarial gain (loss) (2,500) (439)
Contributions by employer 0 1,415
Benefit payments (3,227) (1,542)
Exchange gain (loss) (4,317) 3,441
Ending Balance 48,059 56,304
Defined Benefit Pension Plans [member]    
Disclosure of fair value of plan assets [line items]    
Beginning Balance 2,429 2,054
Interest income on plan assets 73 86
Actuarial gain (loss) (64) (55)
Contributions by employer 0 219
Benefit payments (399) (57)
Exchange gain (loss) (175) 182
Ending Balance 1,864 2,429
Supplemental pension plan [member]    
Disclosure of fair value of plan assets [line items]    
Beginning Balance 53,875 49,306
Interest income on plan assets 1,726 1,983
Actuarial gain (loss) (2,436) (384)
Contributions by employer 0 1,196
Benefit payments (2,828) (1,485)
Exchange gain (loss) (4,142) 3,259
Ending Balance $ 46,195 $ 53,875