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Share-based compensation - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of shares reserved for future grants of stock options and share appreciation rights (in shares) 4,311,729  
Liabilities from share-based payment transactions $ 18,382 $ 52,794
SARs and TSARs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Maximum term 7 years  
Fair value of share-based payment transactions $ 7,800  
Liabilities from share-based payment transactions 7,300  
Intrinsic value of liabilities from share-based payment transactions $ 500  
Weighted average remaining vesting period 1 year 7 months  
Expense from share-based payment transactions with employees $ 8,700 1,200
Expense from share-based payment transactions with employees, effect of change in share price $ 13,700 7,800
Vesting rate 33.33%  
Performance Share Units (PSU) New Plan    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Maximum term 3 years  
Deferred, Restricted and Performance Share Units    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Fair value of share-based payment transactions $ 14,700  
Liabilities from share-based payment transactions 11,000  
Intrinsic value of liabilities from share-based payment transactions $ 3,700  
Weighted average remaining vesting period 1 year 11 months  
Expense from share-based payment transactions with employees $ 4,500 5,100
Expense from share-based payment transactions with employees, effect of change in share price $ 4,900 8,900
Stock Options    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Maximum term 7 years  
Expense from share-based payment transactions with employees $ 200 $ 400
Minimum | Tranche 2 | Performance Share Units (PSU) New Plan    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share-based compensation arrangement vesting rights, percentage 0.00%  
Maximum | Tranche 2 | Performance Share Units (PSU) New Plan    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share-based compensation arrangement vesting rights, percentage 200.00%