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Income taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Income Tax Disclosure [Abstract]        
Income tax expense (benefit) $ (2,161) $ 6,444 $ 12,205 $ 7,004