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Fair value measurements (Details 3) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 1,871,232 $ 2,093,830
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value $ 1,893,442 $ 2,238,548