Restructuring charges - Costs Incurred and Anticipated (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 6.3	$ 0.5	$ 8.8	$ 0.6
2018 Plan
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.2
Cumulative costs					$ 7.0
Anticipated total costs	7.0		7.0
2018 Plan | USA
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.1
Cumulative costs					5.7
Anticipated total costs	5.7		5.7
2018 Plan | Employee termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.2
Cumulative costs					6.9
Anticipated total costs	6.9		6.9
2018 Plan | Employee termination costs | USA
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.1
Cumulative costs					5.6
Anticipated total costs	5.6		5.6
2018 Plan | Employee termination costs | Other:
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.1
Cumulative costs					1.3
Anticipated total costs	1.3		1.3
2018 Plan | Other exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.0
Cumulative costs					0.1
Anticipated total costs	0.1		0.1
2018 Plan | Other exit costs | USA
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.0
Cumulative costs					$ 0.1
Anticipated total costs	$ 0.1		$ 0.1