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Income taxes - Other (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Income taxes    
Expense for the year $ 42.3 $ 34.1
Adjustment in respect of prior years (3.2) 6.0
Current income tax expense 39.1 40.1
Origination and reversal of temporary differences in the current year 20.2 (7.7)
U.S. Tax Reform impact   2.0
Impact of changes in tax rate (1.0) 2.9
Change in unrecognized deductible temporary differences (0.1)  
Change in unrecognized deductible temporary differences   19.0
Adjustments in respect of prior years 3.4 (6.2)
Other 0.2  
Deferred income tax expense 22.7 10.0
Total income tax expense $ 61.8 $ 50.1