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Derivative financial instruments (Details) - USD ($)
$ in Thousands
1 Months Ended
Oct. 31, 2004
Dec. 31, 2015
Dec. 31, 2014
Fair value hedge related to long-term debt due in 2020 [Member]      
Derivative financial instruments [Line Items]      
Notional amount   $ 200,000 $ 200,000
Fair value of interest rate swaps   4,842 8,067
Cash flow hedge, interest rate swaps 2004 [Member]      
Derivative financial instruments [Line Items]      
Pre-tax loss on cash flow hedge $ 17,877    
Senior notes due 2020 [Member]      
Derivative financial instruments [Line Items]      
Decrease in debt due to fair value adjustments   $ (4,842) $ (8,067)