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Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2018
Mar. 31, 2018
Income Tax Disclosure        
Beginning Balance $ 7.5 $ 7.2 $ 7.5 $ 7.3
Addition for tax positions 0.2 0.3 0.3 0.2
Ending Balance $ 7.7 $ 7.5 $ 7.8 $ 7.5