Retirement Benefits - Estimated OPEB Future Ben Pmts (Details) $ in Millions	Dec. 31, 2020 USD ($)
Retirement benefits other than pension
Estimated Future Benefits Payments [Line Items]
2021	$ 40.0
2022	38.7
2023	37.8
2024	36.5
2025	35.3
2026 through 2029	158.4
UMWA Plans
Estimated Future Benefits Payments [Line Items]
2021	29.7
2022	29.1
2023	28.8
2024	28.1
2025	27.5
2026 through 2029	126.7
Black Lung and Other Plans
Estimated Future Benefits Payments [Line Items]
2021	10.3
2022	9.6
2023	9.0
2024	8.4
2025	7.8
2026 through 2029	$ 31.7

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- Definition

Amount of benefit for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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Amount of benefit for defined benefit plan expected to be paid in next fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in fifth fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in fourth fiscal year following current fiscal year.

+ References

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Amount of benefit for defined benefit plan expected to be paid in third fiscal year following current fiscal year.

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Amount of benefit for defined benefit plan expected to be paid in second fiscal year following current fiscal year.

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