Employee plans - Components of Net Periodic Benefit Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.0	$ 0.0	$ 0.0
Interest cost	197.5	186.6	211.3
Expected return on plan assets	(218.2)	(230.6)	(235.2)
Amortization of prior service credit	(2.5)	(2.5)	(2.5)
Recognized net actuarial loss	116.6	125.1	126.4
Curtailment gain	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic pension/benefit cost (income)	93.4	78.6	100.0
Pension Plans | International Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.8	3.2	5.1
Interest cost	68.3	67.3	72.8
Expected return on plan assets	(104.6)	(114.4)	(127.5)
Amortization of prior service credit	(2.5)	(3.7)	(2.4)
Recognized net actuarial loss	34.2	42.3	49.8
Curtailment gain	(0.1)	0.0	(5.4)
Settlement loss	1.2	6.4	0.0
Net periodic pension/benefit cost (income)	(0.7)	1.1	(7.6)
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.5	0.6	0.5
Interest cost	4.8	4.8	5.6
Expected return on plan assets	(0.4)	(0.4)	(0.5)
Amortization of prior service credit	(1.7)	(1.6)	(0.7)
Recognized net actuarial loss	0.7	1.0	0.8
Net periodic pension/benefit cost (income)	$ 3.9	$ 4.4	$ 5.7