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Purchase commitments (Details 3) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Purchase commitments due      
2020 $ 405,535    
2021 250,266    
2022 184,225    
2023 123,166    
2024 87,297    
Thereafter 678,432    
Amounts purchased under various commitments $ 686,500 $ 548,000 $ 516,100
Maximum      
Long-term Purchase Commitment [Line Items]      
Unrecorded Unconditional Purchase Obligation, Term 41 years