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Share-based compensation - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
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 3,953,471  
Liabilities from share-based payment transactions $ 111,405 $ 53,725
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 $ 69,800  
Liabilities from share-based payment transactions 65,200  
Intrinsic value of liabilities from share-based payment transactions $ 4,600  
Weighted average remaining vesting period 1 year 6 months  
Expense from share-based payment transactions with employees $ 45,100 26,900
Expense from share-based payment transactions with employees, effect of change in share price 37,800 20,000
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 55,900  
Liabilities from share-based payment transactions 46,100  
Intrinsic value of liabilities from share-based payment transactions $ 9,800  
Weighted average remaining vesting period 1 year 6 months  
Expense from share-based payment transactions with employees $ 33,000 6,000
Expense from share-based payment transactions with employees, effect of change in share price $ 29,900 2,800
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 $ 500 $ 600