Retirement Benefits - Obligations and Funded Status (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 08, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year		$ 1,145.2	$ 1,066.1
Service cost		9.7	9.9	$ 10.5
Interest cost		38.3	44.5	43.7
Participant contributions		0.7	0.1
Plan amendments		0.3	(0.6)
Plan combinations		1.0	1.4
Acquisitions		132.5
Curtailment		(1.5)	(0.8)
Settlements		(0.7)	(54.5)
Benefits paid		(65.6)	(67.3)
Actuarial losses		141.4	136.7
Foreign currency exchange effects		26.5	9.7
Benefit obligation at end of year		1,427.8	1,145.2	1,066.1
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		914.4	867.2
Return on assets – actual		140.3	143.3
Participant contributions		0.7	0.1
Plan combinations		1.0	1.4
Employer contributions		14.8	14.6
Acquisitions		80.3
Settlements		(0.7)	(53.9)
Benefits paid		(65.6)	(67.3)
Foreign currency exchange effects		17.7	9.0
Fair value of plan assets at end of year		1,102.9	914.4	867.2
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		(324.9)	(230.8)
Current liability, included in accrued liabilities		2.8	1.9
Noncurrent liability		322.1	228.9
Net pension liability		324.9	230.8
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses		(378.1)	(368.0)
Net actuarial gains (losses) arising during the year		(59.4)	(53.0)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)		36.1	45.2
Foreign currency exchange effects		(2.5)	(2.3)
End of year - net gains and losses		(403.9)	(378.1)	(368.0)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit		(0.5)	(1.2)
Prior service credit (cost) from plan amendments during the year		(0.3)	0.6
Reclassification adjustment for amortization of prior service cost included in net income (loss)		0.0	0.1
Foreign currency exchange effects		0.2	0.0
End of year - net prior service (cost) credit		(0.6)	(0.5)	(1.2)
Retirement benefits other than pension
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year		536.7	555.6
Service cost		0.1	0.2	0.2
Interest cost		16.5	21.1	20.3
Plan amendments		0.0	(0.3)
Acquisitions		0.0	0.9
Curtailment		0.0	(0.2)
Benefits paid		(33.1)	(37.8)
Actuarial losses		40.3	(2.6)
Foreign currency exchange effects		(2.5)	(0.2)
Benefit obligation at end of year		558.0	536.7	555.6
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		177.9	181.7
Return on assets – actual		14.1	25.9
Employer contributions		7.4	8.5
Net transfers to (from) plan assets		1.7	(0.4)
Benefits paid		(33.1)	(37.8)
Fair value of plan assets at end of year		168.0	177.9	181.7
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		(390.0)	(358.8)
Current liability, included in accrued liabilities		10.3	11.0
Noncurrent liability		379.7	347.8
Net pension liability		390.0	358.8
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses		(297.3)	(333.8)
Net actuarial gains (losses) arising during the year		(39.2)	15.2
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)		24.8	21.2
Foreign currency exchange effects		1.3	0.1
End of year - net gains and losses		(310.4)	(297.3)	(333.8)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit		29.4	34.2
Prior service credit (cost) from plan amendments during the year		0.0	0.3
Reclassification adjustment for amortization of prior service cost included in net income (loss)		(5.0)	(5.1)
Foreign currency exchange effects		(0.2)	0.0
End of year - net prior service (cost) credit		24.2	29.4	34.2
U.S. Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year		826.8	801.9
Service cost		0.0	0.0	0.0
Interest cost		26.7	34.1	31.9
Participant contributions		0.0	0.0
Plan amendments		0.0	0.0
Plan combinations		0.0	0.0
Settlements		0.0	(53.6)
Benefits paid		(44.0)	(49.1)
Actuarial losses		98.5	93.5
Foreign currency exchange effects		0.0	0.0
Benefit obligation at end of year		908.0	826.8	801.9
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		699.3	686.6
Return on assets – actual		91.2	114.1
Participant contributions		0.0	0.0
Plan combinations		0.0	0.0
Employer contributions		0.6	0.7
Settlements		0.0	(53.0)
Benefits paid		(44.0)	(49.1)
Foreign currency exchange effects		0.0	0.0
Fair value of plan assets at end of year		747.1	699.3	686.6
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		(160.9)	(127.5)
Current liability, included in accrued liabilities		0.6	0.6
Noncurrent liability		160.3	126.9
Net pension liability		160.9	127.5
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses		(296.6)	(306.0)
Net actuarial gains (losses) arising during the year		(53.5)	(29.5)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)		28.6	38.9
Foreign currency exchange effects		0.0	0.0
End of year - net gains and losses		(321.5)	(296.6)	(306.0)
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
U.S. Plans | Actuarial Gains (Losses) - Change in Discount Rate
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		(93.0)
U.S. Plans | Actuarial Gains (Losses) - Change in Discount Rate | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			(96.0)
U.S. Plans | Actuarial Gains (Losses) - Change in Census Data
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		(5.0)
U.S. Plans | Actuarial Gains (Losses) - Asset Return Different from Expected
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		45.0
U.S. Plans | Actuarial Gains (Losses) - Asset Return Different from Expected | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			63.0
U.S. Plans | Actuarial Gains (Losses) - Change of Mortality Tables | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			5.0
U.S. Plans | Total
Changes in the benefit obligation [Roll Forward]
Settlements	$ (54.0)
Non-U.S. Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year		318.4	264.2
Service cost		9.7	9.9	10.5
Interest cost		11.6	10.4	11.8
Participant contributions		0.7	0.1
Plan amendments		0.3	(0.6)
Plan combinations		1.0	1.4
Acquisitions		132.5
Curtailment		(1.5)	(0.8)
Settlements		(0.7)	(0.9)
Benefits paid		(21.6)	(18.2)
Actuarial losses		42.9	43.2
Foreign currency exchange effects		26.5	9.7
Benefit obligation at end of year		519.8	318.4	264.2
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		215.1	180.6
Return on assets – actual		49.1	29.2
Participant contributions		0.7	0.1
Plan combinations		1.0	1.4
Employer contributions		14.2	13.9
Acquisitions		80.3
Settlements		(0.7)	(0.9)
Benefits paid		(21.6)	(18.2)
Foreign currency exchange effects		17.7	9.0
Fair value of plan assets at end of year		355.8	215.1	180.6
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		(164.0)	(103.3)
Current liability, included in accrued liabilities		2.2	1.3
Noncurrent liability		161.8	102.0
Net pension liability		164.0	103.3
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses		(81.5)	(62.0)
Net actuarial gains (losses) arising during the year		(5.9)	(23.5)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)		7.5	6.3
Foreign currency exchange effects		(2.5)	(2.3)
End of year - net gains and losses		(82.4)	(81.5)	(62.0)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit		(0.5)	(1.2)
Prior service credit (cost) from plan amendments during the year		(0.3)	0.6
Reclassification adjustment for amortization of prior service cost included in net income (loss)		0.0	0.1
Foreign currency exchange effects		0.2	0.0
End of year - net prior service (cost) credit		(0.6)	(0.5)	(1.2)
Non-U.S. Plans | Actuarial Gains (Losses) - Change in Discount Rate
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		(45.0)	(37.0)
Non-U.S. Plans | Actuarial Gains (Losses) - Asset Return Different from Expected
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		37.0	19.0
UMWA Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year		424.6	479.1
Service cost		0.0	0.0	0.0
Interest cost		12.7	17.3	17.1
Plan amendments		0.0	0.0
Curtailment		0.0	0.0
Benefits paid		(25.7)	(29.3)
Actuarial losses		28.5	(42.5)
Foreign currency exchange effects		0.0	0.0
Benefit obligation at end of year		440.1	424.6	479.1
Changes in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		177.9	181.7
Return on assets – actual		14.1	25.9
Net transfers to (from) plan assets		1.7	(0.4)
Benefits paid		(25.7)	(29.3)
Fair value of plan assets at end of year		168.0	177.9	181.7
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		(272.1)	(246.7)
Current liability, included in accrued liabilities		0.0	0.0
Noncurrent liability		272.1	246.7
Net pension liability		272.1	246.7
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses		(219.2)	(290.9)
Net actuarial gains (losses) arising during the year		(27.4)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)		16.5	16.6
Foreign currency exchange effects		0.0	0.0
End of year - net gains and losses		(230.1)	(219.2)	(290.9)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit		28.0	32.7
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)		(4.7)	(4.7)
Foreign currency exchange effects		0.0	0.0
End of year - net prior service (cost) credit		23.3	28.0	32.7
UMWA Plans | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			55.1
UMWA Plans | Actuarial Gains (Losses) - Change in Discount Rate
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		(37.0)
UMWA Plans | Actuarial Gains (Losses) - Change in Discount Rate | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			(50.0)
UMWA Plans | Actuarial Gains (Losses) - Change in Census Data
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			4.0
UMWA Plans | Actuarial Gains (Losses) - Asset Return Different from Expected | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			13.0
UMWA Plans | Actuarial Gains (Losses) - Change of Mortality Tables | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			6.0
UMWA Plans | Actuarial Gains (Losses) - Change in Claims Experience
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		10.0	53.0
UMWA Plans | Actuarial Gains (Losses) - Excise Tax Update
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			29.0
Black Lung and Other Plans
Changes in the benefit obligation [Roll Forward]
Benefit obligation at beginning of year		112.1	76.5
Service cost		0.1	0.2	0.2
Interest cost		3.8	3.8	3.2
Plan amendments		0.0	(0.3)
Acquisitions		0.0	0.9
Curtailment		0.0	(0.2)
Benefits paid		(7.4)	(8.5)
Actuarial losses		11.8	39.9
Foreign currency exchange effects		(2.5)	(0.2)
Benefit obligation at end of year		117.9	112.1	76.5
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	8.5
Benefits paid		(7.4)	(8.5)
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		(117.9)	(112.1)
Current liability, included in accrued liabilities		10.3	11.0
Noncurrent liability		107.6	101.1
Net pension liability		117.9	112.1
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Beginning of year - net gains and losses		(78.1)	(42.9)
Net actuarial gains (losses) arising during the year		(11.8)
Reclassification adjustment for amortization of prior actuarial losses included in net income (loss)		8.3	4.6
Foreign currency exchange effects		1.3	0.1
End of year - net gains and losses		(80.3)	(78.1)	(42.9)
Benefit plan prior service cost recognized in accumulated other comprehensive income (loss):
Beginning of year - net prior service (cost) credit		1.4	1.5
Prior service credit (cost) from plan amendments during the year		0.0	0.3
Reclassification adjustment for amortization of prior service cost included in net income (loss)		(0.3)	(0.4)
Foreign currency exchange effects		(0.2)	0.0
End of year - net prior service (cost) credit		0.9	1.4	$ 1.5
Black Lung and Other Plans | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			(39.9)
Black Lung and Other Plans | Actuarial Gains (Losses) - Change in Discount Rate
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		(8.0)
Black Lung and Other Plans | Actuarial Gains (Losses) - Change in Discount Rate | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			(5.0)
Black Lung and Other Plans | Actuarial Gains (Losses) - Change in Census Data
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		(5.0)
Black Lung and Other Plans | Actuarial Gains (Losses) - Change in Census Data | ASU 2017-07
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			(8.0)
Black Lung and Other Plans | Actuarial Gains (Losses) - Change of Mortality Tables
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year			3.0
Black Lung and Other Plans | Actuarial Gains (Losses) - Claims Assumptions Updates
Benefit plan net actuarial losses recognized in accumulated other comprehensive income (loss):
Net actuarial gains (losses) arising during the year		$ 3.0	$ (27.0)