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Asset Impairment (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Impaired Long-Lived Assets Held and Used [Line Items]      
Asset impairment charges $ 0.0 $ 1,487.4 $ 270.2
Asset impairment of property, plant and equipment and mine development assets 0.0 1,487.4 261.2
Impairment to unallocated coal reserves 69.3 69.2  
Impairment of investment in equity security 0.0 0.0 $ 9.0
Assets subject to recoverability review 500.0    
Powder River Basin Mining      
Impaired Long-Lived Assets Held and Used [Line Items]      
Asset impairment charges $ 1,418.1    
Asset impairment of property, plant and equipment and mine development assets   1,393.7  
Operating lease right-of-use assets impairment   19.9  
Impairment of contract-based intangible assets   4.5  
Other U.S. Thermal Mining      
Impaired Long-Lived Assets Held and Used [Line Items]      
Asset impairment charges   $ 192.0