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Income taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Income Tax Disclosure [Abstract]        
Income tax (expense) benefit $ (3,321) $ 848 $ (4,465) $ 3,237