Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Provision for income taxes (in millions)	$ 14.7		$ 23.0		$ 37.1	$ 53.0
Effective tax rate	67.40%		54.90%		51.00%	(514.60%)
Tax Cuts and Jobs Act of 2017
Operating Loss Carryforwards [Line Items]
Tax Reform Act Tax Expense Charge		$ 2.3		$ 92.0
Foreign tax credit amount		1.3		$ 31.1
State tax amount		$ 0.2