Stock-based compensation - expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Other information about options outstanding
Expense	$ 11.4	$ 8.4
Stock options
Other information about options outstanding
Expense	4.6	3.9
RSUs
Other information about options outstanding
Expense	5.7	4.9
Optionholder awards
Other information about options outstanding
Expense	0.0	0.3
Other
Other information about options outstanding
Expense	1.1	(0.7)
Equity
Other information about options outstanding
Expense	10.8	9.2
Liability
Other information about options outstanding
Expense	$ 0.6	$ (0.8)