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Fair value measurements (Details 3) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Mar. 31, 2015
Carrying amount [Member]      
Fair value, balance sheet grouping [Line Items]      
Long-term debt $ 1,927,054 $ 1,865,163 $ 2,183,644
Fair value [Member]      
Fair value, balance sheet grouping [Line Items]      
Long-term debt, fair value $ 1,996,415 $ 1,887,373 $ 2,334,339