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Income taxes - Income Before Income Taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Income Tax Disclosure [Abstract]        
Income tax expense (benefit) $ (12,329) $ 8,474 $ (43,562) $ 22,975
Effective tax rate 10.60% 31.10% 12.60% 24.60%