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Fair value measurements (Details 4) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Carrying amount [Member]      
Fair value, balance sheet grouping [Line Items]      
Long-term debt $ 1,740,552 $ 1,790,159 $ 1,901,948
Fair value [Member]      
Fair value, balance sheet grouping [Line Items]      
Long-term debt, fair value $ 1,846,811 $ 1,841,885 $ 2,047,339