Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Provision for income taxes (in millions)	$ 22.9	$ 58.9	$ 59.2	$ 3.5
Effective tax rate	49.90%	161.40%	47.50%	(3500.00%)