Retirement Benefits - Obligations and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year	$ 1,113.2	$ 1,107.6
Service cost	11.3	10.5	$ 10.4
Interest cost	50.3	48.6	47.2
Participant contributions	0.4	0.2
Plan amendments	0.0	(0.2)
Plan combinations	0.6	0.4
Acquisition	2.5	0.0
Settlements	(1.0)	(9.3)
Benefits paid	(65.7)	(60.5)
Actuarial (gains) losses	65.8	29.5
Foreign currency exchange effects	14.4	(13.6)
Benefit obligation at end of year	1,191.8	1,113.2	1,107.6
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	901.9	886.2
Return on assets – actual	113.2	72.5
Participant contributions	0.4	0.2
Employer contributions	17.5	15.5
Settlements	(1.0)	(9.3)
Benefits paid	(65.7)	(60.5)
Foreign currency exchange effects	13.2	(3.1)
Fair value of plan assets at end of year	980.1	901.9	886.2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(211.7)	(211.3)
Current liability, included in accrued liabilities	2.9	2.5
Noncurrent liability	208.8	208.8
Net pension liability	211.7	211.3
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses	(403.8)	(428.7)
Net actuarial gains (losses) arising during the year	(15.8)	(21.1)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)	33.9	33.1
Foreign currency exchange effects	(6.1)	12.9
End of year - net gains and losses	(391.8)	(403.8)	(428.7)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit	(9.2)	(10.8)
Prior service credit (cost) from plan amendments during the year	0.0	0.2
Reclassification adjustment for amortization of prior service cost included in net income (loss)	1.1	1.0
Foreign currency exchange effects	(0.2)	0.4
End of year - net prior service (cost) credit	(8.3)	(9.2)	(10.8)
Amortization of losses	(31.9)	(30.0)	(36.2)
Amortization of prior service cost	1.1	1.0	0.9
Net periodic postretirement cost	33.4	29.1	36.3
Retirement benefits other than pension
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year	510.3	492.9
Service cost	0.1	0.0	0.2
Interest cost	21.6	21.6	19.9
Plan amendments	(1.1)	0.0
Curtailment	(2.4)	0.0
Benefits paid	(40.9)	(40.0)
Actuarial (gains) losses	101.7	35.1
Foreign currency exchange effects	0.0	0.7
Benefit obligation at end of year	589.3	510.3	492.9
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	217.6	227.4
Return on assets – actual	34.6	22.2
Employer contributions	7.9	8.0
Benefits paid	(40.9)	(40.0)
Fair value of plan assets at end of year	219.2	217.6	227.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(370.1)	(292.7)
Current liability, included in accrued liabilities	7.3	6.6
Noncurrent liability	362.8	286.1
Net pension liability	370.1	292.7
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses	(295.2)	(287.6)
Net actuarial gains (losses) arising during the year	(81.2)	(30.4)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)	23.6	20.5
Foreign currency exchange effects	0.0	2.3
End of year - net gains and losses	(352.8)	(295.2)	(287.6)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit	39.8	43.0
Prior service credit (cost) from plan amendments during the year	1.1	0.0
Reclassification adjustment for amortization of prior service cost included in net income (loss)	(3.0)	(2.9)
Foreign currency exchange effects	0.1	(0.3)
End of year - net prior service (cost) credit	38.0	39.8	43.0
Amortization of losses	(23.6)	(20.5)	(18.6)
Amortization of prior service cost	(2.9)	(2.9)	(2.8)
Curtailment (gain)	(0.1)	0.0	(0.1)
Net periodic postretirement cost	25.8	21.7	15.2
U.S. Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year	845.9	844.8
Service cost	0.0	0.0	0.0
Interest cost	35.1	37.1	36.0
Participant contributions	0.0	0.0
Plan amendments	0.0	0.0
Plan combinations	0.0	0.0
Acquisition	0.0	0.0
Settlements	0.0	0.0
Benefits paid	(49.1)	(47.8)
Actuarial (gains) losses	58.4	11.8
Foreign currency exchange effects	0.0	0.0
Benefit obligation at end of year	890.3	845.9	844.8
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	728.5	721.4
Return on assets – actual	97.1	54.3
Participant contributions	0.0	0.0
Employer contributions	0.7	0.6
Settlements	0.0	0.0
Benefits paid	(49.1)	(47.8)
Foreign currency exchange effects	0.0	0.0
Fair value of plan assets at end of year	777.2	728.5	721.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(113.1)	(117.4)
Current liability, included in accrued liabilities	0.7	0.9
Noncurrent liability	112.4	116.5
Net pension liability	113.1	117.4
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses	(320.9)	(333.7)
Net actuarial gains (losses) arising during the year	(14.6)	(12.1)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)	26.6	24.9
Foreign currency exchange effects	0.0	0.0
End of year - net gains and losses	(308.9)	(320.9)	(333.7)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit	0.0	0.0
Prior service credit (cost) from plan amendments during the year	0.0	0.0
Reclassification adjustment for amortization of prior service cost included in net income (loss)	0.0	0.0
Foreign currency exchange effects	0.0	0.0
End of year - net prior service (cost) credit	0.0	0.0	0.0
Amortization of losses	(26.6)	(24.9)	(31.2)
Amortization of prior service cost	0.0	0.0	0.0
Net periodic postretirement cost	8.4	7.4	12.6
Non-U.S. Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year	267.3	262.8
Service cost	11.3	10.5	10.4
Interest cost	15.2	11.5	11.2
Participant contributions	0.4	0.2
Plan amendments	0.0	(0.2)
Plan combinations	0.6	0.4
Acquisition	2.5	0.0
Settlements	(1.0)	(9.3)
Benefits paid	(16.6)	(12.7)
Actuarial (gains) losses	7.4	17.7
Foreign currency exchange effects	14.4	(13.6)
Benefit obligation at end of year	301.5	267.3	262.8
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	173.4	164.8
Return on assets – actual	16.1	18.2
Participant contributions	0.4	0.2
Employer contributions	16.8	14.9
Settlements	(1.0)	(9.3)
Benefits paid	(16.6)	(12.7)
Foreign currency exchange effects	13.2	(3.1)
Fair value of plan assets at end of year	202.9	173.4	164.8
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(98.6)	(93.9)
Current liability, included in accrued liabilities	2.2	1.6
Noncurrent liability	96.4	92.3
Net pension liability	98.6	93.9
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses	(82.9)	(95.0)
Net actuarial gains (losses) arising during the year	(1.2)	(9.0)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)	7.3	8.2
Foreign currency exchange effects	(6.1)	12.9
End of year - net gains and losses	(82.9)	(82.9)	(95.0)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit	(9.2)	(10.8)
Prior service credit (cost) from plan amendments during the year	0.0	0.2
Reclassification adjustment for amortization of prior service cost included in net income (loss)	1.1	1.0
Foreign currency exchange effects	(0.2)	0.4
End of year - net prior service (cost) credit	(8.3)	(9.2)	(10.8)
Amortization of losses	(5.3)	(5.1)	(5.0)
Amortization of prior service cost	1.1	1.0	0.9
Net periodic postretirement cost	25.0	21.7	23.7
UMWA Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year	444.2	433.1
Service cost	0.0	0.0	0.0
Interest cost	18.4	18.9	17.1
Plan amendments	0.0	0.0
Curtailment	0.0	0.0
Benefits paid	(33.5)	(31.7)
Actuarial (gains) losses	84.4	23.9
Foreign currency exchange effects	0.0	0.0
Benefit obligation at end of year	513.5	444.2	433.1
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	217.6	227.4
Return on assets – actual	34.6	22.2
Employer contributions	0.5	(0.3)
Benefits paid	(33.5)	(31.7)
Fair value of plan assets at end of year	219.2	217.6	227.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(294.3)	(226.6)
Current liability, included in accrued liabilities	0.0	0.0
Noncurrent liability	294.3	226.6
Net pension liability	294.3	226.6
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses	(263.0)	(261.8)
Net actuarial gains (losses) arising during the year	(66.3)	(19.2)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)	19.5	18.0
Foreign currency exchange effects	0.0	0.0
End of year - net gains and losses	(309.8)	(263.0)	(261.8)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit	41.9	46.5
Prior service credit (cost) from plan amendments during the year	0.0	0.0
Reclassification adjustment for amortization of prior service cost included in net income (loss)	(4.6)	(4.6)
Foreign currency exchange effects	0.0	0.0
End of year - net prior service (cost) credit	37.3	41.9	46.5
Amortization of losses	(19.5)	(18.0)	(15.5)
Amortization of prior service cost	(4.6)	(4.6)	(4.6)
Curtailment (gain)	0.0	0.0	0.0
Net periodic postretirement cost	16.8	14.8	7.4
Black Lung and Other Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year	66.1	59.8
Service cost	0.1	0.0
Interest cost	3.2	2.7
Plan amendments	(1.1)	0.0
Curtailment	(2.4)	0.0
Benefits paid	(7.4)	(8.3)
Actuarial (gains) losses	17.3	11.2
Foreign currency exchange effects	0.0	0.7
Benefit obligation at end of year	75.8	66.1	59.8
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	0.0	0.0
Return on assets – actual	0.0	0.0
Employer contributions	7.4	8.3
Benefits paid	(7.4)	(8.3)
Fair value of plan assets at end of year	0.0	0.0	0.0
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(75.8)	(66.1)
Current liability, included in accrued liabilities	7.3	6.6
Noncurrent liability	68.5	59.5
Net pension liability	75.8	66.1
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses	(32.2)	(25.8)
Net actuarial gains (losses) arising during the year	(14.9)	(11.2)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)	4.1	2.5
Foreign currency exchange effects	0.0	2.3
End of year - net gains and losses	(43.0)	(32.2)	(25.8)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit	(2.1)	(3.5)
Prior service credit (cost) from plan amendments during the year	1.1	0.0
Reclassification adjustment for amortization of prior service cost included in net income (loss)	1.6	1.7
Foreign currency exchange effects	0.1	(0.3)
End of year - net prior service (cost) credit	0.7	(2.1)	(3.5)
Black Lung and Other Plans, Medical
Changes in the benefit obligation [Roll Forward]
Service cost	0.1	0.0	0.2
Interest cost	3.2	2.7	2.8
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Amortization of losses	(4.1)	(2.5)	(3.1)
Amortization of prior service cost	1.7	1.7	1.8
Curtailment (gain)	(0.1)	0.0	(0.1)
Net periodic postretirement cost	$ 9.0	$ 6.9	$ 7.8