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Purchase commitments (Details 4) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Purchase commitments due      
2018 $ 360,751    
2019 215,005    
2020 162,424    
2021 135,334    
2022 99,068    
Thereafter 773,820    
Amounts purchased under various commitments $ 516,100 $ 539,300 $ 842,100
Maximum [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Term 43 years