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Other Events (Details) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended 18 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2019
Jun. 18, 2019
Other Commercial Events [Line Items]                
Restructuring Charges $ 16,500,000 $ 400,000   $ 23,000,000.0 $ 600,000      
Provision for North Goonyella equipment loss 0 0   0 24,700,000 $ 66,400,000    
Insurance recovery related to North Goonyella equipment losses $ 0 0   $ 0 (125,000,000.0)      
Interest in unincorporated joint venture project 50.00%     50.00%        
North Goonyella Mine                
Other Commercial Events [Line Items]                
Provision for North Goonyella equipment loss         24,700,000   $ 149,600,000  
Containment and idling costs $ 11,300,000 $ 28,400,000   $ 21,400,000 65,300,000      
Insurance recovery related to North Goonyella equipment losses     $ 125,000,000   $ 125,000,000.0      
Deductible Amount     $ 50,000,000          
North Goonyella Mine | Mine Carrying Value                
Other Commercial Events [Line Items]                
Costs Related to North Goonyella Events       $ 300,000,000        
PRB Colorado Joint Venture [Member]                
Other Commercial Events [Line Items]                
Joint Venture Ownership Percentage by Parent               66.50%
Joint Venture Ownership Percentage by Noncontrolling Owners               33.50%