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Share-based payments - Summary of Options Granted Priced Using Black Scholes for SARs, Tandem Awards, Converted Options, Matching Stock Options and Monte Carlo Option Pricing Model (Detail)
4 Months Ended 12 Months Ended
Dec. 17, 2020
yr
$ / shares
Sep. 14, 2020
yr
$ / shares
Jun. 30, 2020
yr
$ / shares
Sep. 30, 2019
yr
$ / shares
Dec. 31, 2019
yr
$ / shares
Dec. 31, 2018
yr
$ / shares
2016 Tandem Awards [Member]            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Exercise price       $ 18.30 $ 23.82 $ 16.65
Underlying share price       $ 16.94 $ 67.87 $ 27.21
Volatility       78.00% 73.00% 64.00%
Time period (years) | yr       2.10 1.25 5.00
Risk free rate       2.04% 1.59% 2.77%
Dividend yield       0.00% 0.00% 0.00%
Combined probability of exit events       60.00 80.00 25.00
Matching Stock Options [Member]            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Exercise price     $ 10.00      
Underlying share price     $ 15.15      
Volatility     75.00%      
Time period (years) | yr     5.5      
Risk free rate     0.29%      
Dividend yield     0.00%      
Share Appreciation Rights [Member]            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Exercise price         $ 1.12 $ 1.12
Underlying share price         $ 67.87 $ 27.21
Volatility         73.00% 64.00%
Time period (years) | yr         1.25 5.00
Risk free rate         1.59% 2.77%
Dividend yield         0.00% 0.00%
Combined probability of exit events         80.00 25.00
Additional Grants Under 2020 Plan [Member]            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Exercise price $ 9.70 $ 10.00 $ 10.00      
Underlying share price $ 9.70 $ 9.16 $ 15.15      
Volatility 84.00% 79.00% 75.00%      
Time period (years) | yr 6.0 6.2 7.0      
Risk free rate 0.49% 0.37% 0.29%      
Dividend yield 0.00% 0.00% 0.00%      
Performance Based Options [Member]            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Exercise price   $ 10.00 $ 10.00      
Underlying share price   $ 9.16 $ 15.15      
Volatility   78.00% 79.00%      
Time period (years) | yr   6.7 7.0      
Risk free rate   0.67% 0.66%      
Dividend yield   0.00% 0.00%      
Converted Options [Member]            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Exercise price     $ 2.47      
Underlying share price     $ 15.15      
Volatility     75.00%      
Time period (years) | yr     5.6      
Risk free rate     0.29%      
Dividend yield     0.00%