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Other Events (Details) T in Millions, $ in Millions, $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 28, 2017 USD ($) T | Nov. 27, 2017 USD ($) | Jul. 31, 2018 USD ($) | Jun. 30, 2018 USD ($) T | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Apr. 01, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 AUD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Feb. 06, 2018 | Nov. 30, 2016 USD ($)
Successor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs Related to North Goonyella Events |  |  |  |  |  |  |  |  |  |  |  |  | $ 58.0 |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  | $ 17.1 | $ 49.3 |  |  |  |  | $ 0.0 |  | 66.4 |  |  |  |  |
Restructuring charges | $ 3.0 |  |  |  |  |  |  |  |  |  | 7.6 |  | 1.2 |  |  |  |  |
Coal Reserves, in tons | T | 345.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclamation and Mine Shutdown Provision | $ 34.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Asset Retirement Obligations |  |  |  |  |  |  |  |  |  |  | 12.1 |  | 5.7 |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  | 17.9 |  | 76.4 |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  | 20.7 | $ 30.6 | $ 83.1 |  |  | 84.0 |  | $ 48.2 |  |  |  |  |
Interest in unincorporated joint venture project |  |  |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% | 50.00% |  |  |  |
Insurance Claim Limit |  |  |  |  |  |  |  |  |  |  |  |  | $ 125.0 |  |  |  |  |
Deductible Amount |  |  |  |  |  |  |  |  |  |  |  |  | 50.0 |  |  |  |  |
Successor | Springfield Reclamation Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  | 31.2 |  |  | 31.2 |  | 0.0 |  |  |  |  |
Gain (Loss) on Disposition of Assets | $ 31.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Successor | Burton Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  | $ 52.2 |  |  | 52.2 |  | 0.0 |  |  |  |  |
Gain (Loss) on Disposition of Assets |  | $ 52.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Successor | Red Mountain Joint Venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  | 20.0 |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  |  |  |  |  | 7.1 |  |  |  |  |
Successor | Queensland’s Bowen Basin |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 37.0 |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  |  |  |  |  | 20.6 |  |  |  |  |
Successor | DTA and PBGC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage of equity method investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |
Successor | New Mexico/Colorado Mining Tenement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Contract Termination |  |  |  |  |  |  |  |  |  |  | $ 20.0 |  |  |  |  |  |  |
Successor | Stanmore Coal Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Coal Reserves, in tons | T |  |  |  | 23.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  | $ 22.0 |  |  |  |  |  |  |  |  | 7.0 |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  | $ 20.5 |  |  | $ 20.5 |  |  |  |  |  |  |  |  |  |  |  |
Successor | Lenton Joint Venture | Burton Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Planned Divestiture, Agreed Upon Proceeds from Divestiture |  | 11.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Asset Retirement Obligations |  | 40.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in Restricted Cash |  | $ 30.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Successor | Leased Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  |  |  |  |  |  |  |  |  | 50.4 |  |  |  |  |
Successor | Owned Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  |  |  |  |  |  |  |  |  | $ 16.0 |  |  |  |  |
Predecessor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  |  |  |  |  | $ 0.0 |  |  |  |  |  | $ 0.0 |  |  |
Restructuring charges |  |  |  |  |  |  |  |  | 0.0 |  |  |  |  |  | 15.5 |  |  |
Increase (Decrease) in Asset Retirement Obligations |  |  |  |  |  |  |  |  | 10.2 |  |  |  |  |  | 13.1 |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  | 24.3 |  |  |  |  |  | 144.4 |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  | 22.8 |  |  |  |  |  | 23.2 |  |  |
Predecessor | Springfield Reclamation Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  |  | 0.0 |  |  |  |  |  | 0.0 |  |  |
Predecessor | Burton Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  |  | 0.0 |  |  |  |  |  | 0.0 |  |  |
Predecessor | DTA and PBGC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage of equity method investment |  |  |  |  |  |  |  |  |  | 37.50% |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  | $ 20.5 |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  | $ 19.7 |  |  |  |  |  |  |  |  |
Predecessor | Metropolitan Collieries Pty Ltd |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Planned Divestiture, Agreed Upon Proceeds from Divestiture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200.0
Predecessor | Priairie State Campus |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 57.1 |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.2 |  |  |
Predecessor | Nonstrategic Australian mining tenement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 64.1 |  |  |
Predecessor | Olive Downs South Mining Tenement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.8 |  |  |
Predecessor | New Mexico/Colorado Mining Tenement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 358.0 |  |
Predecessor | Prarie State Energy Campus |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Undivided Interest Percent Of New Electricity Generation Project |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.06% |  |  |
Scenario, Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs Related to North Goonyella Events |  |  |  |  |  |  |  |  |  |  |  | $ 285.0 |  |  |  |  |  |
Scenario, Forecast | Leased Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs Related to North Goonyella Events |  |  |  |  |  |  |  |  |  |  |  | $ 16.0 |  |  |  |  |  |

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