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Share-based compensation - Summary of Performance Share Unit Plan (Detail) - PSU Plan [member] - Options granted from June 2017 [member]
9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Outstanding, beginning of year 92,424 163,129
Granted 376,310 144,211
Vested   (22,929)
Forfeited (4,173) (191,987)
Outstanding, end of year 464,561 92,424