Restructuring charges - Restructuring and related activities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0.9	$ 0.6	$ 9.7	$ 1.2
2020 Plan
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.7		10.3
Anticipated total costs	10.3		10.3
Employee termination costs | 2020 Plan | USA
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.5		6.2
Anticipated total costs	6.2		6.2
Employee termination costs | 2020 Plan | Canada
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.2		3.3
Anticipated total costs	3.3		3.3
Employee termination costs | 2020 Plan | Other:
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.0		0.8
Anticipated total costs	$ 0.8		$ 0.8