Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.8	$ 2.1
Interest cost on projected benefit obligation	12.5	9.0
Return on assets – expected	(14.6)	(15.0)
Amortization of losses	0.9	6.3
Settlement loss	0.1	0.4
Net periodic pension cost	0.7	2.8
Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.0
Interest cost on projected benefit obligation	4.3	3.6
Return on assets – expected	(2.6)	(3.3)
Amortization of losses	2.8	5.0
Amortization of prior service credit	(2.7)	(1.2)
Net periodic pension cost	1.9	4.1
UMWA Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost on projected benefit obligation	3.0	2.7
Return on assets – expected	(2.6)	(3.3)
Amortization of losses	1.7	3.1
Amortization of prior service credit	(2.7)	(1.2)
Net periodic pension cost	(0.6)	1.3
Black Lung and Other Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.0
Interest cost on projected benefit obligation	1.3	0.9
Return on assets – expected	0.0	0.0
Amortization of losses	1.1	1.9
Amortization of prior service credit	0.0	0.0
Net periodic pension cost	2.5	2.8
U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost on projected benefit obligation	8.1	5.7
Return on assets – expected	(11.8)	(12.1)
Amortization of losses	0.5	5.8
Settlement loss	0.0	0.0
Net periodic pension cost	(3.2)	(0.6)
Non-U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.8	2.1
Interest cost on projected benefit obligation	4.4	3.3
Return on assets – expected	(2.8)	(2.9)
Amortization of losses	0.4	0.5
Settlement loss	0.1	0.4
Net periodic pension cost	$ 3.9	$ 3.4