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Other Events (Details)
$ in Millions
 12 Months Ended
Oct. 25, 2023
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2023
AUD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Other Commercial Events [Line Items]          
Mine Reclamation and Closing Liability, Noncurrent   $ 4,100,000      
Revenue related to excess port and rail capacity agreement   19,200,000      
Other Noncash Income   9,600,000   $ 0 $ 0
Proceeds From Collaborative Arrangement Related to Port and Rail Capacity     $ 10.0    
Collaborative Arrangement, Revenue Not from Contract with Customer   30,000,000      
Collaborative Arrangement Related to Port and Rail Capacity, Revenue Not from Contract with Customer   6,700,000      
Wards Well coal deposit          
Other Commercial Events [Line Items]          
Asset Acquisition, Price of Acquisition, Expected $ 136,000,000        
Contingent royalty $ 200,000,000        
Millennium Mine          
Other Commercial Events [Line Items]          
Discontinued Operation, Gain (Loss) on Disposal   26,100,000      
Wilkie Creek Mine          
Other Commercial Events [Line Items]          
Discontinued Operation, Gain (Loss) on Disposal   24,600,000      
North Antelope Rochelle Mine          
Other Commercial Events [Line Items]          
Storm Damage Provision   12,200,000      
North Antelope Rochelle Mine | Materials and supplies inventories          
Other Commercial Events [Line Items]          
Storm Damage Provision   4,000,000      
North Antelope Rochelle Mine | Buildings and equipment          
Other Commercial Events [Line Items]          
Provision for buildings and equipment   1,000,000      
North Antelope Rochelle Mine | Incremental repair costs          
Other Commercial Events [Line Items]          
Storm Damage Provision   7,200,000      
Shoal Creek          
Other Commercial Events [Line Items]          
Provision for loss related to fire   28,700,000      
Loss limits on insurance claim, amount   125,000,000      
Deductible   50,000,000      
Shoal Creek | Longwall and development and other costs          
Other Commercial Events [Line Items]          
Provision for loss related to fire   17,800,000      
Shoal Creek | Equipment deemed inoperable          
Other Commercial Events [Line Items]          
Provision for loss related to fire   $ 10,900,000