Retirement Benefits - Retirement Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Total
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7.6	$ 8.1	$ 9.1
Interest cost on projected benefit obligation	50.5	36.0	33.2
Return on assets – expected	(58.3)	(61.4)	(59.8)
Amortization of losses	3.4	26.2	40.6
Curtailment gain	0.0	(0.5)	(0.8)
Settlement loss	0.0	3.2	3.3
Net periodic pension cost (credit)	3.2	11.6	25.6
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.3	0.1	0.1
Interest cost on projected benefit obligation	16.4	14.0	13.0
Return on assets – expected	(10.3)	(13.2)	(12.3)
Amortization of losses	9.9	17.3	26.5
Amortization of prior service cost	(11.1)	(4.9)	(5.0)
Net periodic pension cost (credit)	5.2	13.3	22.3
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0
Interest cost on projected benefit obligation	11.1	10.3	9.8
Return on assets – expected	(10.3)	(13.2)	(12.3)
Amortization of losses	5.1	10.0	17.5
Amortization of prior service cost	(11.0)	(4.6)	(4.7)
Net periodic pension cost (credit)	(5.1)	2.5	10.3
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.3	0.1	0.1
Interest cost on projected benefit obligation	5.3	3.7	3.2
Return on assets – expected	0.0	0.0	0.0
Amortization of losses	4.8	7.3	9.0
Amortization of prior service cost	(0.1)	(0.3)	(0.3)
Net periodic pension cost (credit)	10.3	10.8	12.0
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0
Interest cost on projected benefit obligation	32.4	22.9	21.1
Return on assets – expected	(47.2)	(48.7)	(47.4)
Amortization of losses	1.6	24.2	34.0
Curtailment gain	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic pension cost (credit)	(13.2)	(1.6)	7.7
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	7.6	8.1	9.1
Interest cost on projected benefit obligation	18.1	13.1	12.1
Return on assets – expected	(11.1)	(12.7)	(12.4)
Amortization of losses	1.8	2.0	6.6
Curtailment gain	0.0	(0.5)	(0.8)
Settlement loss	0.0	3.2	3.3
Net periodic pension cost (credit)	$ 16.4	$ 13.2	$ 17.9