Retirement benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.9	$ 2.0	$ 3.7	$ 4.1
Interest cost on projected benefit obligation	12.6	9.0	25.1	18.0
Return on assets – expected	(14.5)	(15.7)	(29.1)	(30.7)
Amortization of losses	0.8	6.8	1.7	13.1
Settlement loss	0.2	0.1	0.3	0.5
Net periodic pension cost	1.0	2.2	1.7	5.0
Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.2	0.1
Interest cost on projected benefit obligation	4.0	3.5	8.3	7.1
Return on assets – expected	(2.6)	(3.3)	(5.2)	(6.6)
Amortization of losses	2.4	4.0	5.2	9.0
Amortization of prior service credit	(2.8)	(1.2)	(5.5)	(2.4)
Net periodic pension cost	1.1	3.1	3.0	7.2
UMWA Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0	0.0
Interest cost on projected benefit obligation	2.7	2.6	5.7	5.3
Return on assets – expected	(2.6)	(3.3)	(5.2)	(6.6)
Amortization of losses	1.2	2.2	2.9	5.3
Amortization of prior service credit	(2.8)	(1.1)	(5.5)	(2.3)
Net periodic pension cost	(1.5)	0.4	(2.1)	1.7
Black Lung and Other Plans | Retirement benefits other than pensions
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.2	0.1
Interest cost on projected benefit obligation	1.3	0.9	2.6	1.8
Return on assets – expected	0.0	0.0	0.0	0.0
Amortization of losses	1.2	1.8	2.3	3.7
Amortization of prior service credit	0.0	(0.1)	0.0	(0.1)
Net periodic pension cost	2.6	2.7	5.1	5.5
U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0	0.0
Interest cost on projected benefit obligation	8.1	5.7	16.2	11.4
Return on assets – expected	(11.8)	(12.3)	(23.6)	(24.4)
Amortization of losses	0.4	6.3	0.9	12.1
Settlement loss	0.0	0.0	0.0	0.0
Net periodic pension cost	(3.3)	(0.3)	(6.5)	(0.9)
Non-U.S. Plans | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.9	2.0	3.7	4.1
Interest cost on projected benefit obligation	4.5	3.3	8.9	6.6
Return on assets – expected	(2.7)	(3.4)	(5.5)	(6.3)
Amortization of losses	0.4	0.5	0.8	1.0
Settlement loss	0.2	0.1	0.3	0.5
Net periodic pension cost	$ 4.3	$ 2.5	$ 8.2	$ 5.9