Retirement Benefits - Retirement Cost (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total
Defined Benefit Plan Disclosure [Line Items]
Service cost			$ 9.7	$ 9.9	$ 10.5
Interest cost on projected benefit obligation			38.3	44.5	43.7
Return on assets – expected			(58.3)	(61.0)	(64.7)
Amortization of losses			33.7	23.8	32.3
Amortization of prior service cost			0.0	0.1	0.5
Curtailment gain			(1.5)
Settlement loss			2.4	21.4	1.7
Net periodic pension cost			24.3	38.7	24.0
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.2	0.2
Interest cost on projected benefit obligation			16.5	21.1	20.3
Return on assets – expected			(13.0)	(13.3)	(16.7)
Amortization of losses			24.8	21.2	26.1
Amortization of prior service cost			(5.0)	(5.0)	(3.5)
Curtailment gain			0.0	(0.1)	0.0
Net periodic pension cost			23.4	24.1	26.4
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.0	0.0	0.0
Interest cost on projected benefit obligation			12.7	17.3	17.1
Return on assets – expected			(13.0)	(13.3)	(16.7)
Amortization of losses			16.5	16.6	20.3
Amortization of prior service cost			(4.7)	(4.7)	(4.6)
Curtailment gain			0.0	0.0	0.0
Net periodic pension cost			11.5	15.9	16.1
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.2	0.2
Interest cost on projected benefit obligation			3.8	3.8	3.2
Return on assets – expected			0.0	0.0	0.0
Amortization of losses			8.3	4.6	5.8
Amortization of prior service cost			(0.3)	(0.3)	1.1
Curtailment gain			0.0	(0.1)	0.0
Net periodic pension cost			11.9	8.2	10.3
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.0	0.0	0.0
Interest cost on projected benefit obligation			26.7	34.1	31.9
Return on assets – expected			(46.2)	(50.7)	(53.6)
Amortization of losses			28.6	19.6	27.7
Amortization of prior service cost			0.0	0.0	0.0
Settlement loss			0.0	19.3	0.0
Net periodic pension cost			9.1	22.3	6.0
U.S. Plans | Total
Defined Benefit Plan Disclosure [Line Items]
Settlement loss	$ (19.3)	$ (19.3)
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost			9.7	9.9	10.5
Interest cost on projected benefit obligation			11.6	10.4	11.8
Return on assets – expected			(12.1)	(10.3)	(11.1)
Amortization of losses			5.1	4.2	4.6
Amortization of prior service cost			0.0	0.1	0.5
Curtailment gain			(1.5)
Settlement loss			2.4	2.1	1.7
Net periodic pension cost			$ 15.2	$ 16.4	$ 18.0