Employee plans - Funded Status of the Plan and Amounts Recognized in Consolidated Balance Sheet (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	$ 187.5	$ 136.2	$ 147.6
Other accrued liabilities	(11.7)	(13.6)
Long-term postretirement liabilities	(1,286.1)	(1,960.2)
Pension Plans | U.S. Plans
Change in projected/accumulated benefit obligation
Benefit obligation at beginning of year	4,755.6	4,558.0
Service cost	0.0	0.0	0.0
Interest cost	162.5	197.5	186.6
Plan participants’ contributions	0.0	0.0
Plan curtailment	0.0	0.0
Plan settlement	(277.3)	0.0
Actuarial loss	253.9	357.7
Benefits paid	(349.4)	(357.6)
Foreign currency translation and other adjustments	0.0	0.0
Benefit obligation at end of year	4,545.3	4,755.6	4,558.0
Change in plan assets
Fair value of plan assets at beginning of year	3,334.2	3,112.8
Actual return on plan assets	347.2	505.2
Employer contribution	793.1	73.8
Plan participants’ contributions	0.0	0.0
Plan settlement	(277.3)	0.0
Benefits paid	(349.4)	(357.6)
Foreign currency translation adjustments	0.0	0.0
Fair value of plan assets at end of year	3,847.8	3,334.2	3,112.8
Funded status at end of year	(697.5)	(1,421.4)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	27.2	0.0
Other accrued liabilities	(6.1)	(6.8)
Long-term postretirement liabilities	(718.6)	(1,414.6)
Total funded status	(697.5)	(1,421.4)
Accumulated other comprehensive loss, net of tax
Net loss	2,510.4	2,672.7
Prior service credit	(32.3)	(34.8)
Accumulated benefit obligation	4,545.3	4,755.6
Pension Plans | International Plans
Change in projected/accumulated benefit obligation
Benefit obligation at beginning of year	3,143.8	2,829.5
Service cost	2.8	2.8	3.2
Interest cost	53.4	68.3	67.3
Plan participants’ contributions	1.1	1.3
Plan curtailment	(1.6)	(1.6)
Plan settlement	0.0	(3.5)
Actuarial loss	226.5	284.1
Benefits paid	(119.0)	(118.1)
Foreign currency translation and other adjustments	161.0	81.0
Benefit obligation at end of year	3,468.0	3,143.8	2,829.5
Change in plan assets
Fair value of plan assets at beginning of year	2,816.4	2,539.4
Actual return on plan assets	254.7	300.0
Employer contribution	33.1	30.1
Plan participants’ contributions	1.1	1.3
Plan settlement	0.0	(3.5)
Benefits paid	(119.0)	(118.1)
Foreign currency translation adjustments	143.1	67.2
Fair value of plan assets at end of year	3,129.4	2,816.4	2,539.4
Funded status at end of year	(338.6)	(327.4)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	160.3	135.9
Other accrued liabilities	(0.2)	(0.2)
Long-term postretirement liabilities	(498.7)	(463.1)
Total funded status	(338.6)	(327.4)
Accumulated other comprehensive loss, net of tax
Net loss	1,116.9	1,067.2
Prior service credit	(45.9)	(46.4)
Accumulated benefit obligation	3,360.4	3,035.3
Other Postretirement Benefit Plans
Change in projected/accumulated benefit obligation
Benefit obligation at beginning of year	95.7	96.2
Service cost	0.5	0.5	0.6
Interest cost	4.4	4.8	4.8
Plan participants’ contributions	2.3	2.7
Actuarial loss	(13.8)	1.0
Benefits paid	(8.8)	(8.9)
Foreign currency translation and other adjustments	(0.1)	(0.6)
Benefit obligation at end of year	80.2	95.7	96.2
Change in plan assets
Fair value of plan assets at beginning of year	6.9	7.8
Actual return on plan assets	(0.4)	(0.2)
Employer contribution	6.0	5.5
Plan participants’ contributions	2.3	2.7
Benefits paid	(8.8)	(8.9)
Fair value of plan assets at end of year	6.0	6.9	$ 7.8
Funded status at end of year	(74.2)	(88.8)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	0.0	0.3
Other accrued liabilities	(5.4)	(6.6)
Long-term postretirement liabilities	(68.8)	(82.5)
Total funded status	(74.2)	(88.8)
Accumulated other comprehensive loss, net of tax
Net loss	(3.0)	11.1
Prior service credit	$ (4.9)	$ (6.6)