Retirement Benefits - Retirement Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11.3	$ 10.5	$ 10.4
Interest cost on projected benefit obligation	50.3	48.6	47.2
Return on assets – expected	(63.2)	(64.1)	(64.1)
Amortization of losses	31.9	30.0	36.2
Amortization of prior service cost	1.1	1.0	0.9
Settlement loss	2.0	3.1	5.7
Net periodic pension cost	33.4	29.1	36.3
Total | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	33.4	29.1	35.2
Total | Discontinued operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	0.0	0.0	1.1
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.0	0.2
Interest cost on projected benefit obligation	21.6	21.6	19.9
Return on assets – expected	(16.5)	(17.5)	(20.6)
Amortization of losses	23.6	20.5	18.6
Amortization of prior service cost	(2.9)	(2.9)	(2.8)
Net periodic pension cost	25.8	21.7	15.2
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0
Interest cost on projected benefit obligation	18.4	18.9	17.1
Return on assets – expected	(16.5)	(17.5)	(20.6)
Amortization of losses	19.5	18.0	15.5
Amortization of prior service cost	(4.6)	(4.6)	(4.6)
Net periodic pension cost	16.8	14.8	7.4
Black Lung and Other Plans, Medical
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.0	0.2
Interest cost on projected benefit obligation	3.2	2.7	2.8
Return on assets – expected	0.0	0.0	0.0
Amortization of losses	4.1	2.5	3.1
Amortization of prior service cost	1.7	1.7	1.8
Net periodic pension cost	9.0	6.9	7.8
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0
Interest cost on projected benefit obligation	35.1	37.1	36.0
Return on assets – expected	(53.3)	(54.6)	(54.6)
Amortization of losses	26.6	24.9	31.2
Amortization of prior service cost	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic pension cost	8.4	7.4	12.6
U.S. Plans | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	8.4	7.4	12.6
U.S. Plans | Discontinued operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	0.0	0.0	0.0
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	11.3	10.5	10.4
Interest cost on projected benefit obligation	15.2	11.5	11.2
Return on assets – expected	(9.9)	(9.5)	(9.5)
Amortization of losses	5.3	5.1	5.0
Amortization of prior service cost	1.1	1.0	0.9
Settlement loss	2.0	3.1	5.7
Net periodic pension cost	25.0	21.7	23.7
Non-U.S. Plans | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	25.0	21.7	22.6
Non-U.S. Plans | Discontinued operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost	$ 0.0	$ 0.0	$ 1.1