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Financial Instruments and Other Guarantees Textuals (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Jun. 30, 2017
Apr. 01, 2017
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2016
Apr. 03, 2017
Guarantee Obligations [Line Items]                      
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities $ 1,317.0             $ 1,136.8 $ 1,317.0    
Secured Debt | Accounts Receivable Securitization Program, April 3, 2020                      
Guarantee Obligations [Line Items]                      
Cash collateral required 0.0               0.0    
Successor                      
Guarantee Obligations [Line Items]                      
Asset retirement obligations               691.1      
Cash collateral in support of reclamation obligations or activities               205.2      
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities 1,589.8               1,589.8    
Letters of credit outstanding, amount 245.0               245.0    
Interest expense 36.5 $ 38.2 $ 38.3 $ 36.3 $ 42.5 $ 41.4   119.7 149.3    
Provision for North Goonyella equipment loss 17.1 $ 49.3           0.0 66.4    
Successor | Obligations                      
Guarantee Obligations [Line Items]                      
Restricted cash collateral 0.0             323.1 0.0    
Successor | Secured Debt | Accounts Receivable Securitization Program, April 3, 2020                      
Guarantee Obligations [Line Items]                      
Long-term debt 0.0               0.0    
Exit facility, maximum borrowing capacity                     $ 250.0
Letters of credit outstanding, amount 137.1               137.1    
Interest expense               5.3 5.2    
Successor | Obligations                      
Guarantee Obligations [Line Items]                      
Restricted cash collateral 0.0             $ 40.1 0.0    
Successor | U.S Mining                      
Guarantee Obligations [Line Items]                      
Asset retirement obligations 750.2               750.2    
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities 1,317.0               1,317.0    
Letters of credit outstanding, amount $ 147.3               $ 147.3    
Predecessor                      
Guarantee Obligations [Line Items]                      
Asset retirement obligations             $ 664.2     $ 758.8  
Interest expense             32.9     298.6  
Provision for North Goonyella equipment loss             0.0     0.0  
Predecessor | Secured Debt | Accounts Receivable Securitization Program, April 3, 2020                      
Guarantee Obligations [Line Items]                      
Interest expense             $ 2.0     $ 8.2