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Share-based compensation - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
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 4,530,865  
Liabilities from share-based payment transactions $ 52,794 $ 111,405
SARs and TSARs    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Maximum term 7 years  
Share-based compensation arrangement vesting rights, percentage 33.33%  
Fair value of share-based payment transactions $ 18,900  
Liabilities from share-based payment transactions 17,300  
Intrinsic value of liabilities from share-based payment transactions $ 1,600  
Weighted average remaining vesting period 1 year 7 months  
Expense from share-based payment transactions with employees $ 1,200 45,100
Expense from share-based payment transactions with employees, effect of change in share price 7,800 37,800
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 36,600  
Liabilities from share-based payment transactions 35,300  
Intrinsic value of liabilities from share-based payment transactions $ 1,300  
Weighted average remaining vesting period 1 year 5 months  
Expense from share-based payment transactions with employees $ 5,100 33,000
Expense from share-based payment transactions with employees, effect of change in share price $ 8,900 29,900
Stock Options    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Maximum term 7 years  
Share-based compensation arrangement vesting rights, percentage 33.33%  
Expense from share-based payment transactions with employees $ 400 $ 500