Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Provision (benefit) for income taxes (in millions)	$ (43.2)	$ 12.7	$ (55.4)	$ 22.4
Effective tax rate	158.80%	47.40%	151.40%	43.90%