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Commitments and Contingencies (Details)
AUD in Millions, $ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended 336 Months Ended
Apr. 01, 2017
USD ($)
Dec. 31, 2017
AUD
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2042
Dec. 31, 2017
USD ($)
Mar. 16, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2016
AUD
Long-term Purchase Commitment [Line Items]                    
Annual payments on coal reserves 2013 to 2016 $ 247.9   $ 247.9              
Annual Coal Reserve Payments Pursuant To Belle Ayr North Lease     42.1 $ 42.1 $ 42.1          
Annual Coal Reserve Payments Pursuant To Caballo West Lease     28.6 28.6 28.6          
Annual true up payments for the excess of the $1.10 bid price versus $0.95 under the transferred lease     $ 3.9 $ 3.9 $ 3.9          
Predecessor | Berenergy [Member]                    
Long-term Purchase Commitment [Line Items]                    
Litigation Settlement Damages Awarded to Plaintiff             $ 0.9      
Loss Contingency Accrual                 $ 13.1  
Predecessor | Wilkie Creek [Member]                    
Long-term Purchase Commitment [Line Items]                    
Settlement charges total                 $ 9.9 AUD 13.0
Predecessor | Debtors vs. Blue Tee [Member]                    
Long-term Purchase Commitment [Line Items]                    
Bankruptcy Claims, Amount of Tax Refunds to be Foregone to Settle Claims               $ 11.0    
Bankruptcy Claims, Amount to be Transferred to Settle Claims               12.0    
Bankruptcy Claims, Amount to be Paid to Settle Claims               $ 20.0    
Successor                    
Long-term Purchase Commitment [Line Items]                    
Take-or-pay obligations             1,336.5      
Take-or-pay Obligations Due In One Year             182.2      
Successor | Capital Additions [Member]                    
Long-term Purchase Commitment [Line Items]                    
Purchase commitments for capital expenditures             159.0      
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months             $ 41.9      
Successor | Monto Coal Pty Limited [Member]                    
Long-term Purchase Commitment [Line Items]                    
Loss Contingency, Damages Sought, Value | AUD   AUD 15.6                
LossContingencyDamagesSoughtValueMax | AUD   AUD 1,800.0                
Ownership Percentage In Subsidiaries             51.00%      
Scenario, Forecast                    
Long-term Purchase Commitment [Line Items]                    
Take-or-pay Arrangement Terms Years (Maximum)           25