Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.2	$ 1.8
Interest cost on projected benefit obligation	12.2	12.5
Return on assets – expected	(14.5)	(14.6)
Amortization of losses	2.1	0.9
Settlement loss	0.3	0.1
Net periodic pension cost (credit)	2.3	0.7
Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1
Interest cost on projected benefit obligation	3.9	4.3
Return on assets – expected	(2.5)	(2.6)
Amortization of losses	2.4	2.8
Amortization of prior service credit	(2.8)	(2.7)
Net periodic pension cost (credit)	1.1	1.9
Retirement benefits other than pensions | UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost on projected benefit obligation	2.6	3.0
Return on assets – expected	(2.5)	(2.6)
Amortization of losses	1.3	1.7
Amortization of prior service credit	(2.8)	(2.7)
Net periodic pension cost (credit)	(1.4)	(0.6)
Retirement benefits other than pensions | Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1
Interest cost on projected benefit obligation	1.3	1.3
Return on assets – expected	0.0	0.0
Amortization of losses	1.1	1.1
Amortization of prior service credit	0.0	0.0
Net periodic pension cost (credit)	2.5	2.5
U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost on projected benefit obligation	7.7	8.1
Return on assets – expected	(11.6)	(11.8)
Amortization of losses	1.5	0.5
Settlement loss	0.0	0.0
Net periodic pension cost (credit)	(2.4)	(3.2)
Non-U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.2	1.8
Interest cost on projected benefit obligation	4.5	4.4
Return on assets – expected	(2.9)	(2.8)
Amortization of losses	0.6	0.4
Settlement loss	0.3	0.1
Net periodic pension cost (credit)	$ 4.7	$ 3.9