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Purchase commitments (Details 4) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Purchase commitments due      
2019 $ 418,106    
2020 215,069    
2021 169,716    
2022 115,884    
2023 84,268    
Thereafter 622,383    
Amounts purchased under various commitments $ 548,000 $ 516,100 $ 539,300
Maximum [Member]      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Term 42 years