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Revenues - Revenues by Contract Duration (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Apr. 01, 2017
Jun. 30, 2018
Jun. 30, 2017
Apr. 01, 2017
Jun. 30, 2018
Dec. 31, 2017
Successor            
Disaggregation of Revenue [Line Items]            
Revenues   $ 1,309.4 $ 1,258.3   $ 2,772.1  
Unrealized Gain (Loss) on Price Risk Fair Value Hedging Instruments   48.1 9.4   9.5  
Gain (Loss) on Price Risk Fair Value Hedge Ineffectiveness   9.6 7.7   29.5  
Successor | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues   1,059.7 1,025.7   2,292.8  
Successor | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues   279.1 180.8   441.4  
Successor | Other            
Disaggregation of Revenue [Line Items]            
Revenues   (29.4) 51.8   37.9  
Successor | Operating Segments | Powder River Basin Mining            
Disaggregation of Revenue [Line Items]            
Revenues   321.5 365.4   710.8  
Successor | Operating Segments | Powder River Basin Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues   310.8 326.7   654.2  
Successor | Operating Segments | Powder River Basin Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues   10.7 34.4   56.5  
Successor | Operating Segments | Powder River Basin Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues   0.0 4.3   0.1  
Successor | Operating Segments | Midwestern U.S. Mining            
Disaggregation of Revenue [Line Items]            
Revenues   197.5 194.9   399.2  
Successor | Operating Segments | Midwestern U.S. Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues   194.1 187.3   381.7  
Successor | Operating Segments | Midwestern U.S. Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues   3.4 7.5   17.4  
Successor | Operating Segments | Midwestern U.S. Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues   0.0 0.1   0.1  
Successor | Operating Segments | Western U.S. Mining            
Disaggregation of Revenue [Line Items]            
Revenues   139.6 125.4   283.3  
Successor | Operating Segments | Western U.S. Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues   126.5 122.7   253.8  
Successor | Operating Segments | Western U.S. Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues   8.9 1.1   19.9  
Successor | Operating Segments | Western U.S. Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues   4.2 1.6   9.6  
Successor | Operating Segments | Australian Thermal Mining            
Disaggregation of Revenue [Line Items]            
Revenues   267.4 239.2   468.8  
Successor | Operating Segments | Australian Thermal Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues   173.2 135.6   350.5  
Successor | Operating Segments | Australian Thermal Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues   94.0 103.4   117.7  
Successor | Operating Segments | Australian Thermal Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues   0.2 0.2   0.6  
Successor | Operating Segments | Australian Metallurgical Mining            
Disaggregation of Revenue [Line Items]            
Revenues   417.5 287.8   883.7  
Successor | Operating Segments | Australian Metallurgical Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues   255.1 253.4   652.6  
Successor | Operating Segments | Australian Metallurgical Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues   162.1 34.4   229.9  
Successor | Operating Segments | Australian Metallurgical Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues   0.3 0.0   1.2  
Successor | Corporate, Non-Segment | Trading and Brokerage            
Disaggregation of Revenue [Line Items]            
Revenues   10.0 5.2   30.1  
Successor | Corporate, Non-Segment | Trading and Brokerage | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues   0.0 0.0   0.0  
Successor | Corporate, Non-Segment | Trading and Brokerage | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues   0.0 0.0   0.0  
Successor | Corporate, Non-Segment | Trading and Brokerage | Other            
Disaggregation of Revenue [Line Items]            
Revenues   10.0 5.2   30.1  
Successor | Corporate, Non-Segment | Corporate and Other            
Disaggregation of Revenue [Line Items]            
Revenues [1]   (44.1) 40.4   (3.8)  
Successor | Corporate, Non-Segment | Corporate and Other | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues [1]   0.0 0.0   0.0  
Successor | Corporate, Non-Segment | Corporate and Other | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues [1]   0.0 0.0   0.0  
Successor | Corporate, Non-Segment | Corporate and Other | Other            
Disaggregation of Revenue [Line Items]            
Revenues [1]   (44.1) $ 40.4   (3.8)  
Predecessor            
Disaggregation of Revenue [Line Items]            
Revenues $ 0.0     $ 1,326.2    
Unrealized Gain (Loss) on Price Risk Fair Value Hedging Instruments       16.6    
Gain (Loss) on Price Risk Fair Value Hedge Ineffectiveness       11.1    
Predecessor | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues       1,054.9    
Predecessor | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues       215.2    
Predecessor | Other            
Disaggregation of Revenue [Line Items]            
Revenues       56.1    
Predecessor | Operating Segments | Powder River Basin Mining            
Disaggregation of Revenue [Line Items]            
Revenues       394.3    
Predecessor | Operating Segments | Powder River Basin Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues       357.7    
Predecessor | Operating Segments | Powder River Basin Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues       36.6    
Predecessor | Operating Segments | Powder River Basin Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues       0.0    
Predecessor | Operating Segments | Midwestern U.S. Mining            
Disaggregation of Revenue [Line Items]            
Revenues       193.2    
Predecessor | Operating Segments | Midwestern U.S. Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues       193.2    
Predecessor | Operating Segments | Midwestern U.S. Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues       0.0    
Predecessor | Operating Segments | Midwestern U.S. Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues       0.0    
Predecessor | Operating Segments | Western U.S. Mining            
Disaggregation of Revenue [Line Items]            
Revenues       149.7    
Predecessor | Operating Segments | Western U.S. Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues       129.3    
Predecessor | Operating Segments | Western U.S. Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues       4.2    
Predecessor | Operating Segments | Western U.S. Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues       16.2    
Predecessor | Operating Segments | Australian Thermal Mining            
Disaggregation of Revenue [Line Items]            
Revenues       224.8    
Predecessor | Operating Segments | Australian Thermal Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues       134.1    
Predecessor | Operating Segments | Australian Thermal Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues       90.4    
Predecessor | Operating Segments | Australian Thermal Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues       0.3    
Predecessor | Operating Segments | Australian Metallurgical Mining            
Disaggregation of Revenue [Line Items]            
Revenues       328.9    
Predecessor | Operating Segments | Australian Metallurgical Mining | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues       240.6    
Predecessor | Operating Segments | Australian Metallurgical Mining | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues       84.0    
Predecessor | Operating Segments | Australian Metallurgical Mining | Other            
Disaggregation of Revenue [Line Items]            
Revenues       4.3    
Predecessor | Corporate, Non-Segment | Trading and Brokerage            
Disaggregation of Revenue [Line Items]            
Revenues       15.0    
Predecessor | Corporate, Non-Segment | Trading and Brokerage | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues       0.0    
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues       0.0    
Predecessor | Corporate, Non-Segment | Trading and Brokerage | Other            
Disaggregation of Revenue [Line Items]            
Revenues       15.0    
Predecessor | Corporate, Non-Segment | Corporate and Other            
Disaggregation of Revenue [Line Items]            
Revenues [1]       20.3    
Predecessor | Corporate, Non-Segment | Corporate and Other | One year or longer            
Disaggregation of Revenue [Line Items]            
Revenues [1]       0.0    
Predecessor | Corporate, Non-Segment | Corporate and Other | Short-term contracts            
Disaggregation of Revenue [Line Items]            
Revenues [1]       0.0    
Predecessor | Corporate, Non-Segment | Corporate and Other | Other            
Disaggregation of Revenue [Line Items]            
Revenues [1]       $ 20.3    
Coal Trading Positions [Member] | Successor            
Disaggregation of Revenue [Line Items]            
Derivative Liability, Fair Value, Gross Liability [2]   $ (48.4)     $ (48.4) $ (38.9)
[1] Corporate and Other revenue includes unrealized gains and losses related to mark-to-market activity from financial contract hedge activities intended to secure pricing related to certain coal sales contracts. During the three and six months ended June 30, 2018, such net unrealized losses were $48.1 million and $9.5 million, respectively. During the periods April 2 through June 30, 2017 and January 1 through April 1, 2017, such net unrealized gains were $9.4 million and $16.6 million, respectively. When such gains and losses are realized in connection with recognition of the underlying transaction, they are reclassified to realized gains and losses and are then reflected in Trading and Brokerage revenue (realized losses of $9.6 million and $29.5 million during the three and six months ended June 30, 2018, respectively, and realized losses of $7.7 million and $11.1 million during the periods April 2 through June 30, 2017 and January 1 through April 1, 2017, respectively). At June 30, 2018 and December 31, 2017, the financial contracts’ fair values resulted in net liabilities of $48.4 million and $38.9 million, respectively. Assets and liabilities related to the financial contracts were deemed Level 2 according to the U.S. GAAP fair value hierarchy as the valuations are corroborated by the use of market-based pricing.
[2] Amounts include net liabilities of $48.4 million and $38.9 million at June 30, 2018 and December 31, 2017, respectively, representing the fair value of financial contracts related to hedge activities intended to secure pricing under certain coal sales contracts, as further described in Note 3. “Revenue Recognition.”