Retirement Benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 9.9	$ 10.5	$ 11.3
Interest cost on projected benefit obligation		44.5	43.7	50.3
Return on assets – expected		(61.0)	(64.7)	(63.2)
Amortization of losses		23.8	32.3	31.9
Amortization of prior service cost		0.1	0.5	1.1
Settlement loss		21.4	1.7	2.0
Net periodic pension cost		38.7	24.0	33.4
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.2	0.2	0.1
Interest cost on projected benefit obligation		21.1	20.3	21.6
Return on assets – expected		(13.3)	(16.7)	(16.5)
Amortization of losses		21.2	26.1	23.6
Amortization of prior service cost		(5.0)	(3.5)	(2.9)
Curtailment (gain)		(0.1)	0.0	(0.1)
Net periodic pension cost		24.1	26.4	25.8
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.0	0.0	0.0
Interest cost on projected benefit obligation		17.3	17.1	18.4
Return on assets – expected		(13.3)	(16.7)	(16.5)
Amortization of losses		16.6	20.3	19.5
Amortization of prior service cost		(4.7)	(4.6)	(4.6)
Curtailment (gain)		0.0	0.0	0.0
Net periodic pension cost		15.9	16.1	16.8
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.2	0.2	0.1
Interest cost on projected benefit obligation		3.8	3.2	3.2
Return on assets – expected		0.0	0.0	0.0
Amortization of losses		4.6	5.8	4.1
Amortization of prior service cost		(0.3)	1.1	1.7
Curtailment (gain)		(0.1)	0.0	(0.1)
Net periodic pension cost		8.2	10.3	9.0
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.0	0.0	0.0
Interest cost on projected benefit obligation		34.1	31.9	35.1
Return on assets – expected		(50.7)	(53.6)	(53.3)
Amortization of losses		19.6	27.7	26.6
Amortization of prior service cost		0.0	0.0	0.0
Settlement loss		19.3	0.0	0.0
Net periodic pension cost		22.3	6.0	8.4
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		9.9	10.5	11.3
Interest cost on projected benefit obligation		10.4	11.8	15.2
Return on assets – expected		(10.3)	(11.1)	(9.9)
Amortization of losses		4.2	4.6	5.3
Amortization of prior service cost		0.1	0.5	1.1
Settlement loss		2.1	1.7	2.0
Net periodic pension cost		$ 16.4	$ 18.0	$ 25.0