Retirement Benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Total
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 9.1	$ 9.7	$ 9.9
Interest cost on projected benefit obligation		33.2	38.3	44.5
Return on assets – expected		(59.8)	(58.3)	(61.0)
Amortization of losses		40.6	33.7	23.8
Amortization of prior service cost		0.0	0.0	0.1
Curtailment gain		(0.8)	(1.5)
Settlement loss		3.3	2.4	21.4
Net periodic pension cost		25.6	24.3	38.7
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.1	0.1	0.2
Interest cost on projected benefit obligation		13.0	16.5	21.1
Return on assets – expected		(12.3)	(13.0)	(13.3)
Amortization of losses		26.5	24.8	21.2
Amortization of prior service cost		(5.0)	(5.0)	(5.0)
Curtailment gain		0.0	0.0	(0.1)
Net periodic pension cost		22.3	23.4	24.1
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.0	0.0	0.0
Interest cost on projected benefit obligation		9.8	12.7	17.3
Return on assets – expected		(12.3)	(13.0)	(13.3)
Amortization of losses		17.5	16.5	16.6
Amortization of prior service cost		(4.7)	(4.7)	(4.7)
Curtailment gain		0.0	0.0	0.0
Net periodic pension cost		10.3	11.5	15.9
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.1	0.1	0.2
Interest cost on projected benefit obligation		3.2	3.8	3.8
Return on assets – expected		0.0	0.0	0.0
Amortization of losses		9.0	8.3	4.6
Amortization of prior service cost		(0.3)	(0.3)	(0.3)
Curtailment gain		0.0	0.0	(0.1)
Net periodic pension cost		12.0	11.9	8.2
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.0	0.0	0.0
Interest cost on projected benefit obligation		21.1	26.7	34.1
Return on assets – expected		(47.4)	(46.2)	(50.7)
Amortization of losses		34.0	28.6	19.6
Amortization of prior service cost		0.0	0.0	0.0
Settlement loss		0.0	0.0	19.3
Net periodic pension cost		7.7	9.1	22.3
U.S. Plans | Total
Defined Benefit Plan Disclosure [Line Items]
Settlement loss	$ (19.3)
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		9.1	9.7	9.9
Interest cost on projected benefit obligation		12.1	11.6	10.4
Return on assets – expected		(12.4)	(12.1)	(10.3)
Amortization of losses		6.6	5.1	4.2
Amortization of prior service cost		0.0	0.0	0.1
Curtailment gain		(0.8)	(1.5)
Settlement loss		3.3	2.4	2.1
Net periodic pension cost		$ 17.9	$ 15.2	$ 16.4