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Commitments and contingencies - Narrative (Details)
$ in Thousands, t in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
agreement
t
Dec. 31, 2016
USD ($)
Commitments And Contingencies [Abstract]    
Operating lease payments recognized as expense | $ $ 181,400 $ 165,100
Delivery of vessels currently under construction    
Disclosure of associates [line items]    
Number of time charter agreements | agreement 2  
Future minimum lease payments | $ $ 80,000  
Lease term 15 years  
Within one year    
Disclosure of associates [line items]    
Future minimum lease payments | $ $ 90,820  
Purchase commitment of methanol supply | Within one year    
Disclosure of associates [line items]    
Purchase commitments (in tonnes) | t 0.8  
Purchase commitment of methanol supply | Thereafter    
Disclosure of associates [line items]    
Purchase commitments (in tonnes) | t 1.5  
Atlas Joint Venture    
Disclosure of associates [line items]    
Percentage of marketing rights in production 100.00%  
Percentage of ownership interest in associates 63.10% 63.10%
Atlas Joint Venture | Purchase commitment of methanol supply    
Disclosure of associates [line items]    
Purchase commitments (in tonnes) | t 1.3  
Atlas Joint Venture | Trinidad    
Disclosure of associates [line items]    
Percentage of ownership interest in associates 63.10%  
Atlas Joint Venture | Egypt    
Disclosure of associates [line items]    
Percentage of ownership interest in associates 50.00%