Retirement Benefits - Assumptions (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Pension cost	3.30%	4.40%	3.70%
Benefit obligation at year end	2.40%	3.30%	4.40%
Expected return on assets	8.00%	8.00%	8.00%
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Pension cost	3.30%	4.40%	3.70%
Benefit obligation at year end	2.40%	3.30%	4.40%
Expected return on assets	7.00%	7.00%	7.25%
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Pension cost	3.20%	4.00%	3.50%
Benefit obligation at year end	2.30%	3.20%	4.00%
Expected return on assets	3.28%	5.64%	5.62%
Average rate of increase in salaries - pension cost	2.60%	2.60%	2.60%
Average rate of increase in salaries - benefit obligation at year end	1.90%	2.60%	2.60%
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Pension cost	3.20%	4.30%	3.60%
Benefit obligation at year end	2.30%	3.20%	4.30%
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Pension cost	3.10%	4.20%	3.50%
Benefit obligation at year end	2.20%	3.10%	4.20%