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Investments Equity Method Investments (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Apr. 01, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Predecessor |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity Method Investments |  |  |  |  |  | $ 85.3 |  | $ 85.3 |
(Income) loss from equity affiliates |  |  |  | $ (15.0) | $ (15.0) | (28.8) |  | (16.2) | $ 15.9
Predecessor | Interest In Middlemount Coal Pty Limited [Member] |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity Method Investments |  |  |  |  |  | 84.8 |  | 84.8 |
(Income) loss from equity affiliates |  |  |  | (17.4) |  |  |  | (22.6) | 7.0
Predecessor | Other Equity Method Investments [Member] |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity Method Investments |  |  |  |  |  | $ 0.5 |  | 0.5 |
(Income) loss from equity affiliates |  |  |  | $ 2.4 |  |  |  | $ 6.4 | $ 8.9
Successor |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity Method Investments | $ 83.8 |  |  |  |  |  | $ 83.8 |  |
(Income) loss from equity affiliates | (22.8) | $ (10.5) | $ (15.7) |  |  |  | (49.0) |  |
Successor | Interest In Middlemount Coal Pty Limited [Member] |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity Method Investments | 82.1 |  |  |  |  |  | 82.1 |  |
(Income) loss from equity affiliates |  |  |  |  |  |  | (48.6) |  |
Successor | Other Equity Method Investments [Member] |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |
Equity Method Investments | $ 1.7 |  |  |  |  |  | 1.7 |  |
(Income) loss from equity affiliates |  |  |  |  |  |  | $ (0.4) |  |

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