Employee plans - Funded Status of the Plan and Amounts Recognized in Consolidated Balance Sheet (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	$ 159.7	$ 187.5	$ 136.2
Other accrued liabilities	(12.1)	(11.7)
Long-term postretirement liabilities	(976.2)	(1,286.1)
Pension Plans | U.S. Plans
Change in projected/accumulated benefit obligation
Benefit obligation at beginning of year	4,545.3	4,755.6
Service cost	0.0	0.0	0.0
Interest cost	117.6	162.5	197.5
Plan participants' contributions	0.0	0.0
Plan curtailment	0.0	0.0
Plan settlement	(513.8)	(277.3)
Actuarial loss	(108.4)	253.9
Benefits paid	(331.1)	(349.4)
Foreign currency translation and other adjustments	0.0	0.0
Benefit obligation at end of year	3,709.6	4,545.3	4,755.6
Change in plan assets
Fair value of plan assets at beginning of year	3,847.8	3,334.2
Actual return on plan assets	130.4	347.2
Employer contribution	6.0	793.1
Plan participants' contributions	0.0	0.0
Plan settlement	(513.8)	(277.3)
Benefits paid	(331.1)	(349.4)
Foreign currency translation adjustments	0.0	0.0
Fair value of plan assets at end of year	3,139.3	3,847.8	3,334.2
Funded status at end of year	(570.3)	(697.5)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	33.9	27.2
Other accrued liabilities	(5.9)	(6.1)
Long-term postretirement liabilities	(598.3)	(718.6)
Total funded status	(570.3)	(697.5)
Accumulated other comprehensive loss, net of tax
Net loss (income)	2,047.6	2,510.4
Prior service credit	(29.7)	(32.3)
Accumulated benefit obligation	3,709.6	4,545.3
Pension Plans | International Plans
Change in projected/accumulated benefit obligation
Benefit obligation at beginning of year	3,468.0	3,143.8
Service cost	3.0	2.8	2.8
Interest cost	36.7	53.4	68.3
Plan participants' contributions	1.0	1.1
Plan curtailment	0.0	(1.6)
Plan settlement	(726.8)	0.0
Actuarial loss	2.0	226.5
Benefits paid	(106.5)	(119.0)
Foreign currency translation and other adjustments	(63.0)	161.0
Benefit obligation at end of year	2,614.4	3,468.0	3,143.8
Change in plan assets
Fair value of plan assets at beginning of year	3,129.4	2,816.4
Actual return on plan assets	134.0	254.7
Employer contribution	46.4	33.1
Plan participants' contributions	1.0	1.1
Plan settlement	(726.8)	0.0
Benefits paid	(106.5)	(119.0)
Foreign currency translation adjustments	(45.9)	143.1
Fair value of plan assets at end of year	2,431.6	3,129.4	2,816.4
Funded status at end of year	(182.8)	(338.6)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	125.8	160.3
Other accrued liabilities	(0.1)	(0.2)
Long-term postretirement liabilities	(308.5)	(498.7)
Total funded status	(182.8)	(338.6)
Accumulated other comprehensive loss, net of tax
Net loss (income)	797.6	1,116.9
Prior service credit	(40.2)	(45.9)
Accumulated benefit obligation	2,612.7	3,360.4
Other Postretirement Benefit Plans
Change in projected/accumulated benefit obligation
Benefit obligation at beginning of year	80.2	95.7
Service cost	0.4	0.5	0.5
Interest cost	1.8	4.4	4.8
Plan participants' contributions	1.7	2.3
Amendments	1.2	0.0
Actuarial loss	1.8	(13.8)
Benefits paid	(5.9)	(8.8)
Foreign currency translation and other adjustments	(0.1)	(0.1)
Benefit obligation at end of year	81.1	80.2	95.7
Change in plan assets
Fair value of plan assets at beginning of year	6.0	6.9
Actual return on plan assets	(0.2)	(0.4)
Employer contribution	4.0	6.0
Plan participants' contributions	1.7	2.3
Benefits paid	(5.9)	(8.8)
Fair value of plan assets at end of year	5.6	6.0	$ 6.9
Funded status at end of year	(75.5)	(74.2)
Amounts recognized in the consolidated balance sheets consist of:
Other accrued liabilities	(6.1)	(5.4)
Long-term postretirement liabilities	(69.4)	(68.8)
Total funded status	(75.5)	(74.2)
Accumulated other comprehensive loss, net of tax
Net loss (income)	1.4	(3.0)
Prior service credit	$ (2.1)	$ (4.9)