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Purchase commitments (Details 3) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Purchase commitments due      
2021 $ 458,397    
2022 235,858    
2023 173,306    
2024 124,724    
2025 98,894    
Thereafter 691,613    
Amounts purchased under various commitments $ 666,000 $ 686,500 $ 548,000
Maximum      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Term 39 years