Cost-reduction actions - Reconciliation of Liabilities and Expected Future Payments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Reserve [Roll Forward]
Additional provisions	$ 22.8	$ (0.9)	$ 0.7	$ (2.9)	$ 47.8	$ 46.1	$ 27.5	$ 25.4
Total
Restructuring Reserve [Roll Forward]
Balance at beginning of period				117.8				36.6	$ 89.1	$ 117.8	$ 36.6	$ 33.0
Additional provisions										27.7	131.2	68.3
Payments										(44.4)	(49.3)	(59.3)
Changes in estimates										(8.0)	(8.6)	(4.3)
Translation adjustments										(4.0)	7.9	(1.1)
Balance at end of period	89.1				117.8					89.1	117.8	36.6
In 2019										44.4	49.3	59.3
Total | Scenario, Forecast
Restructuring Reserve [Roll Forward]
Payments									(75.8)
In 2019									75.8
Beyond 2019									13.3
Work-Force Reductions | U.S.
Restructuring Reserve [Roll Forward]
Balance at beginning of period				3.9				1.8	6.1	3.9	1.8	4.2
Additional provisions										5.2	9.4	8.3
Payments										(3.1)	(6.0)	(9.4)
Changes in estimates										0.1	(1.3)	(1.3)
Translation adjustments										0.0	0.0	0.0
Balance at end of period	6.1				3.9					6.1	3.9	1.8
In 2019										3.1	6.0	9.4
Work-Force Reductions | U.S. | Scenario, Forecast
Restructuring Reserve [Roll Forward]
Payments									(5.6)
In 2019									5.6
Beyond 2019									0.5
Work-Force Reductions | International
Restructuring Reserve [Roll Forward]
Balance at beginning of period				109.6				33.4	80.1	109.6	33.4	28.8
Additional provisions										22.5	117.6	58.6
Payments										(39.3)	(41.3)	(49.9)
Changes in estimates										(8.8)	(7.8)	(3.0)
Translation adjustments										(3.9)	7.7	(1.1)
Balance at end of period	80.1				109.6					80.1	109.6	33.4
In 2019										39.3	41.3	49.9
Work-Force Reductions | International | Scenario, Forecast
Restructuring Reserve [Roll Forward]
Payments									(68.8)
In 2019									68.8
Beyond 2019									11.3
Idle Leased Facilities Costs
Restructuring Reserve [Roll Forward]
Balance at beginning of period				$ 4.3				$ 1.4	2.9	4.3	1.4	0.0
Additional provisions										0.0	4.2	1.4
Payments										(2.0)	(2.0)	0.0
Changes in estimates										0.7	0.5	0.0
Translation adjustments										(0.1)	0.2	0.0
Balance at end of period	$ 2.9				$ 4.3					2.9	4.3	1.4
In 2019										$ 2.0	$ 2.0	$ 0.0
Idle Leased Facilities Costs | Scenario, Forecast
Restructuring Reserve [Roll Forward]
Payments									(1.4)
In 2019									1.4
Beyond 2019									$ 1.5