Restructuring charges - Restructuring and related activities (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
2020 Plan
Restructuring Cost and Reserve [Line Items]
Anticipated total costs	9.8		9.8
2020 Plan | Other:
Restructuring Cost and Reserve [Line Items]
Anticipated total costs	0.8		0.8
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
Employee termination costs | 2020 Plan
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6.3		8.6
Employee termination costs | 2020 Plan | USA
Restructuring Cost and Reserve [Line Items]
Restructuring charges	4.7		4.7
Anticipated total costs	5.9		5.9
Employee termination costs | 2020 Plan | Canada
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.8		3.1
Anticipated total costs	3.1		3.1
Employee termination costs | 2020 Plan | Other:
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.8		0.8
Employee termination costs | 2018 Plan
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.2
Cumulative costs					6.9
Anticipated total costs	6.9		6.9
Employee termination costs | 2018 Plan | USA
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.1
Cumulative costs					5.6
Anticipated total costs	5.6		5.6
Employee termination costs | 2018 Plan | Other:
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.1
Cumulative costs					$ 1.3
Anticipated total costs	$ 1.3		$ 1.3