Income taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Provision for income taxes (in millions)	$ 9.7		$ 11.4
Effective tax rate	40.10%		31.10%
Tax Cuts and Jobs Act of 2017
Operating Loss Carryforwards [Line Items]
Tax Reform Act Tax Expense Charge		$ 2.0		$ 92.0
Foreign tax credit amount		1.3		$ 31.1
State tax amount		$ 0.2