Stock-based compensation - expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Other information about options outstanding
Expense	$ 45.8	$ 50.7	$ 44.1
Remaining expense to be recognized	$ 71.0
Weighted average period over which remaining expense will be recognized	1 year 7 months 6 days
Stock options
Other information about options outstanding
Expense	$ 16.0	18.8	16.9
RSUs
Other information about options outstanding
Expense	31.7	26.8	23.8
Optionholder awards
Other information about options outstanding
Expense	0.0	0.0	0.6
Other
Other information about options outstanding
Expense	(1.9)	5.1	2.8
Equity
Other information about options outstanding
Expense	49.1	47.7	42.2
Liability
Other information about options outstanding
Expense	$ (3.3)	$ 3.0	$ 1.9