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Income taxes (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Income Tax Disclosure [Abstract]        
Income tax expense $ (2.3) $ (1.4) $ (1.4) $ (1.1)