Stock-based compensation - stock option rollforward information (Details) - Common stock $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) $ / shares shares
Number of options outstanding
Beginning balance | shares	20.0
Granted (in shares) | shares	1.5
Exercised (in shares) | shares	(1.0)
Forfeited (in shares) | shares	(0.1)
Ending balance | shares	20.4
Weighted average exercise price per outstanding option
Beginning balance | $ / shares	$ 18.80
Granted (in dollars per share) | $ / shares	27.70
Exercised (in dollars per share) | $ / shares	21.40
Forfeited (in dollars per share) | $ / shares	15.88
Ending balance | $ / shares	$ 19.32
Other information about options outstanding
Aggregate intrinsic value | $	$ 197.0
Weighted average remaining term	7 years 6 months
Information about options expected to vest and exercisable
Options expected to vest, number | shares	9.0
Options expected to vest, weighted average exercise price per option | $ / shares	$ 19.80
Options expected to vest, aggregate intrinsic value | $	$ 83.2
Options expected to vest, weighted average remaining term	8 years 7 months 6 days
Options exercisable, number | shares	11.4
Options exercisable, weighted average exercise price per option | $ / shares	$ 18.93
Options exercisable, aggregate intrinsic value | $	$ 113.7
Options exercisable, weighted average remaining term	6 years 7 months 6 days