Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018
Operating Loss Carryforwards [Line Items]
Provision for income taxes (in millions)	$ 12.7		$ 18.6		$ 22.4	$ 30.0
Effective tax rate	47.40%		(20.90%)		43.90%	(57.50%)
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