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Fair value measurements (Details 4) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Fair value, balance sheet grouping [Line Items]      
Long-term debt $ 2,328,266 $ 2,213,130 $ 2,270,290
Carrying amount      
Fair value, balance sheet grouping [Line Items]      
Long-term debt 2,328,266 2,213,130 2,270,290
Fair value      
Fair value, balance sheet grouping [Line Items]      
Long-term debt, fair value $ 2,599,907 $ 2,537,289 $ 2,593,743