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Summary Quarterly Financial Information Textuals (Details) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Nov. 28, 2017
Nov. 27, 2017
Apr. 01, 2017
Jul. 31, 2018
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Apr. 01, 2017
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2016
Successor                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Disposition of Assets           $ 20.7   $ 30.6 $ 83.1         $ 84.0 $ 48.2  
Restructuring Costs and Asset Impairment Charges                           0.0 0.0  
Provision for North Goonyella equipment loss         $ 17.1 49.3               0.0 66.4  
Payments of Debt Restructuring Costs                 6.6              
Income (Loss) from Equity Method Investments           17.2 $ 25.2 22.0 22.8 $ 10.5 $ 15.7     49.0 68.1  
Acquisition costs related to Shoal Creek Mine           4.9               0.0 7.4  
Reorganization items, net               12.8           0.0 (12.8)  
Interest expense         36.5 38.2 38.3 36.3   42.5 $ 41.4     119.7 149.3  
Investment Income, Interest         9.3 10.1 $ 7.0 7.2           5.6 33.6  
Loss on early debt extinguishment                 8.0 $ 12.9       (20.9) (2.0)  
Net mark-to-market adjustment on actuarially determined liabilities         $ 125.5       45.2         (45.2) (125.5)  
Successor | Burton Mine                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Disposition of Assets   $ 52.2                            
Gain (Loss) on Sale of Properties                 52.2         52.2 0.0  
Successor | Springfield Reclamation Liability                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Disposition of Assets $ 31.2                              
Gain (Loss) on Sale of Properties                 $ 31.2         $ 31.2 $ 0.0  
Successor | Iffley Land Transaction                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Disposition of Assets               $ 20.6                
Successor | Stanmore Coal Limited                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Disposition of Assets       $ 20.5   $ 20.5                    
Predecessor                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Disposition of Assets                       $ 22.8       $ 23.2
Restructuring Costs and Asset Impairment Charges                       30.5 $ 30.5     247.9
Provision for North Goonyella equipment loss                       0.0       0.0
Income (Loss) from Equity Method Investments                       15.0 15.0     16.2
Acquisition costs related to Shoal Creek Mine                       0.0       0.0
Reorganization items, net     $ 585.8                 627.2 $ 41.4     159.0
Interest expense                       32.9       298.6
Investment Income, Interest                       2.7       5.7
Loss on early debt extinguishment                       0.0       (29.5)
Net mark-to-market adjustment on actuarially determined liabilities                       0.0       0.0
Predecessor | Burton Mine                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Sale of Properties                       0.0       0.0
Predecessor | Springfield Reclamation Liability                                
Quarterly Financial Information [Line Items]                                
Gain (Loss) on Sale of Properties                       $ 0.0       $ 0.0