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Fair value measurements (Details 3) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Carrying amount [Member]
     
Fair value, balance sheet grouping [Line Items]      
Long-term debt $ 2,210,557 $ 1,854,563 $ 2,011,896
Fair value [Member]
     
Fair value, balance sheet grouping [Line Items]      
Long-term debt, fair value $ 2,332,887 $ 1,912,590 $ 2,106,887