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Asset Impairment (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |
Asset impairment charges | $ 1,487,400,000 | $ 270,200,000 | $ 0
Asset impairment of property, plant and equipment and mine development assets | 1,487,400,000 | 261,200,000 | 0
Impairment to unallocated coal reserves | 69,300,000 | 69,200,000 |
Impairment of investment in equity security | 0 | 9,000,000.0 | $ 0
Assets subject to recoverability review | 1,200,000,000 |  |
Powder River Basin Mining |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |
Asset impairment charges | 1,418,100,000 |  |
Asset impairment of property, plant and equipment and mine development assets | 1,393,700,000 |  |
Operating lease right-of-use assets impairment | 19,900,000 |  |
Impairment of contract-based intangible assets | $ 4,500,000 |  |
Other U.S. Thermal Mining |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |
Asset impairment charges |  | $ 192,000,000.0 |

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- Definition Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228 + Details
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- Definition The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of an intangible asset (excluding goodwill) to fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275 + Details
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- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8A -URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284422-111562 + Details
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