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Other Events (Details)
T in Millions, $ in Millions, $ in Millions
 1 Months Ended 3 Months Ended 6 Months Ended 9 Months Ended
Jun. 30, 2018
USD ($)
T
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Apr. 01, 2017
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2018
AUD ($)
Sep. 20, 2018
USD ($)
Feb. 06, 2018
Mar. 31, 2017
Nov. 30, 2016
USD ($)
Nov. 30, 2015
USD ($)
Successor                          
Other Commercial Events [Line Items]                          
Costs Related to North Goonyella Events     $ 9.0                    
Insurance Claim Limit     125.0                    
Deductible Amount     50.0                    
Provision for North Goonyella equipment loss     49.3 $ 0.0   $ 0.0 $ 49.3            
Proceeds from disposal of assets, net of notes receivable           5.2 69.0            
Net gain on disposal of assets     20.8 0.4   0.9 49.8            
Asset Impairment Charges     $ 0.0 0.0   0.0 $ 0.0            
Interest in unincorporated joint venture project     50.00%       50.00% 50.00%          
Successor | Contract Termination [Member]                          
Other Commercial Events [Line Items]                          
Asset Impairment Charges     $ 0.0 $ 0.0   0.0 $ 0.0            
Successor | Red Mountain Joint Venture                          
Other Commercial Events [Line Items]                          
Proceeds from disposal of assets, net of notes receivable             20.0            
Net gain on disposal of assets             7.1            
Successor | Queensland’s Bowen Basin [Member]                          
Other Commercial Events [Line Items]                          
Proceeds from disposal of assets, net of notes receivable               $ 37.0          
Net gain on disposal of assets             20.6            
Successor | DTA and PBGC                          
Other Commercial Events [Line Items]                          
Equity method investment, ownership percentage                   50.00%      
Successor | Drummond Company, Inc. [Member]                          
Other Commercial Events [Line Items]                          
Business Acquisition, Price of Acquired Entity                 $ 400.0        
Successor | Stanmore Coal Limited [Member]                          
Other Commercial Events [Line Items]                          
Coal Reserves, in tons | T 23.0                        
Proceeds from disposal of assets, net of notes receivable $ 22.0   7.0                    
Net gain on disposal of assets     20.5       $ 20.5            
Successor | Bowie Resource Partners, LLC [Member] | New Mexico And Colorado Assets [Member]                          
Other Commercial Events [Line Items]                          
Gain (Loss) on Contract Termination           $ 20.0              
Successor | Leased Equipment                          
Other Commercial Events [Line Items]                          
Provision for North Goonyella equipment loss     40.2                    
Successor | Owned Equipment                          
Other Commercial Events [Line Items]                          
Provision for North Goonyella equipment loss     $ 9.1                    
Predecessor                          
Other Commercial Events [Line Items]                          
Provision for North Goonyella equipment loss         $ 0.0                
Proceeds from disposal of assets, net of notes receivable         24.3                
Net gain on disposal of assets         22.8                
Asset Impairment Charges         30.5                
Predecessor | Contract Termination [Member]                          
Other Commercial Events [Line Items]                          
Asset Impairment Charges         30.5                
Predecessor | DTA and PBGC                          
Other Commercial Events [Line Items]                          
Equity method investment, ownership percentage                     37.50%    
Proceeds from disposal of assets, net of notes receivable         20.5                
Net gain on disposal of assets         $ 19.7                
Predecessor | Metropolitan Collieries Pty Ltd [Member]                          
Other Commercial Events [Line Items]                          
Planned Divestiture, Agreed Upon Proceeds from Divestiture                       $ 200.0  
Predecessor | Bowie Resource Partners, LLC [Member] | New Mexico And Colorado Assets [Member]                          
Other Commercial Events [Line Items]                          
Planned Divestiture, Agreed Upon Proceeds from Divestiture                         $ 358.0
Scenario, Forecast                          
Other Commercial Events [Line Items]                          
Costs Related to North Goonyella Events   $ 284.0                      
Scenario, Forecast | Leased Equipment                          
Other Commercial Events [Line Items]                          
Costs Related to North Goonyella Events   $ 61.0