Cost-reduction actions - Reconciliation of Liabilities and Expected Future Payments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Additional provisions	$ 23.3	$ 0.2	$ 2.6	$ 2.6	$ 22.8	$ (0.9)	$ 0.7	$ (2.9)	$ 28.7	$ 19.7	$ 146.8
Work-Force Reductions
Restructuring Reserve [Roll Forward]
Balance at beginning of period				86.2				113.5	86.2	113.5	35.2
Additional provisions									25.7	27.7	127.0
Payments									(57.7)	(42.4)	(47.3)
Changes in estimates									(3.6)	(8.7)	(9.1)
Translation adjustments									(0.8)	(3.9)	7.7
Balance at end of period	49.8				86.2				49.8	86.2	113.5
In 2020	47.5								47.5
Beyond 2020	47.5								47.5
Work-Force Reductions | Forecast
Restructuring Reserve [Roll Forward]
In 2020												$ 2.3
Beyond 2020												2.3
Work-Force Reductions | U.S.
Restructuring Reserve [Roll Forward]
Balance at beginning of period				6.1				3.9	6.1	3.9	1.8
Additional provisions									4.6	5.2	9.4
Payments									(4.0)	(3.1)	(6.0)
Changes in estimates									(1.5)	0.1	(1.3)
Translation adjustments									0.0	0.0	0.0
Balance at end of period	5.2				6.1				5.2	6.1	3.9
In 2020	4.8								4.8
Beyond 2020	4.8								4.8
Work-Force Reductions | U.S. | Forecast
Restructuring Reserve [Roll Forward]
In 2020												0.4
Beyond 2020												0.4
Work-Force Reductions | International
Restructuring Reserve [Roll Forward]
Balance at beginning of period				$ 80.1				$ 109.6	80.1	109.6	33.4
Additional provisions									21.1	22.5	117.6
Payments									(53.7)	(39.3)	(41.3)
Changes in estimates									(2.1)	(8.8)	(7.8)
Translation adjustments									(0.8)	(3.9)	7.7
Balance at end of period	44.6				$ 80.1				44.6	$ 80.1	$ 109.6
In 2020	42.7								42.7
Beyond 2020	$ 42.7								$ 42.7
Work-Force Reductions | International | Forecast
Restructuring Reserve [Roll Forward]
In 2020												1.9
Beyond 2020												$ 1.9