Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.1	$ 2.2
Interest cost on projected benefit obligation	9.0	8.2
Return on assets – expected	(15.0)	(14.9)
Amortization of losses	6.3	10.0
Settlement loss	0.4	0.4
Net periodic pension cost	2.8	5.9
Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	3.6	3.3
Return on assets – expected	(3.3)	(3.1)
Amortization of losses	5.0	6.9
Amortization of prior service credit	(1.2)	(1.2)
Net periodic pension cost	4.1	5.9
U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost on projected benefit obligation	5.7	5.3
Return on assets – expected	(12.1)	(11.8)
Amortization of losses	5.8	8.4
Settlement loss	0.0	0.0
Net periodic pension cost	(0.6)	1.9
Non-U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.1	2.2
Interest cost on projected benefit obligation	3.3	2.9
Return on assets – expected	(2.9)	(3.1)
Amortization of losses	0.5	1.6
Settlement loss	0.4	0.4
Net periodic pension cost	3.4	4.0
UMWA Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	2.7	2.5
Return on assets – expected	(3.3)	(3.1)
Amortization of losses	3.1	4.8
Amortization of prior service credit	(1.2)	(1.2)
Net periodic pension cost	1.3	3.0
Black Lung and Other Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	0.9	0.8
Return on assets – expected	0.0	0.0
Amortization of losses	1.9	2.1
Amortization of prior service credit	0.0	0.0
Net periodic pension cost	$ 2.8	$ 2.9