Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Tax Disclosure [Abstract]
Provision (benefit) for income taxes (in millions)	$ 22.7	$ (43.2)	$ 36.3	$ (55.4)
Effective tax rate	45.70%	158.80%	46.10%	151.40%