Retirement Benefits - Retirement Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 10.5	$ 11.3	$ 10.5
Interest cost on projected benefit obligation	43.7	50.3	48.6
Return on assets – expected	(64.7)	(63.2)	(64.1)
Amortization of losses	32.3	31.9	30.0
Amortization of prior service cost	0.5	1.1	1.0
Settlement loss	1.7	2.0	3.1
Net periodic pension cost	24.0	33.4	29.1
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.2	0.1	0.0
Interest cost on projected benefit obligation	20.3	21.6	21.6
Return on assets – expected	(16.7)	(16.5)	(17.5)
Amortization of losses	26.1	23.6	20.5
Amortization of prior service cost	(3.5)	(2.9)	(2.9)
Curtailment (gain)	0.0	(0.1)	0.0
Net periodic pension cost	26.4	25.8	21.7
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0
Interest cost on projected benefit obligation	17.1	18.4	18.9
Return on assets – expected	(16.7)	(16.5)	(17.5)
Amortization of losses	20.3	19.5	18.0
Amortization of prior service cost	(4.6)	(4.6)	(4.6)
Curtailment (gain)	0.0	0.0	0.0
Net periodic pension cost	16.1	16.8	14.8
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.2	0.1	0.0
Interest cost on projected benefit obligation	3.2	3.2	2.7
Return on assets – expected	0.0	0.0	0.0
Amortization of losses	5.8	4.1	2.5
Amortization of prior service cost	1.1	1.7	1.7
Curtailment (gain)	0.0	(0.1)	0.0
Net periodic pension cost	10.3	9.0	6.9
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0
Interest cost on projected benefit obligation	31.9	35.1	37.1
Return on assets – expected	(53.6)	(53.3)	(54.6)
Amortization of losses	27.7	26.6	24.9
Amortization of prior service cost	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic pension cost	6.0	8.4	7.4
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	10.5	11.3	10.5
Interest cost on projected benefit obligation	11.8	15.2	11.5
Return on assets – expected	(11.1)	(9.9)	(9.5)
Amortization of losses	4.6	5.3	5.1
Amortization of prior service cost	0.5	1.1	1.0
Settlement loss	1.7	2.0	3.1
Net periodic pension cost	$ 18.0	$ 25.0	$ 21.7