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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 | 15 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 28, 2017 USD ($) T | Nov. 27, 2017 USD ($) | Jul. 31, 2018 USD ($) | Jun. 30, 2018 USD ($) T | Dec. 31, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Mar. 31, 2018 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 ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) | Jun. 18, 2019 | Feb. 06, 2018 | Nov. 30, 2016 USD ($)
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest in unincorporated joint venture project |  |  |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  |
Insurance Recoveries |  |  |  |  |  | $ 125.0 |  |  |  | $ 0.0 |  | $ 0.0 | $ 125.0 | $ 0.0 |  |  |  |  |  |  |
Gain on formation of United Wambo JV |  |  |  |  | $ (48.1) |  |  |  |  | 0.0 |  | 0.0 | (48.1) | 0.0 |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  | 58.5 | $ 24.7 | $ 17.1 | $ 49.3 |  | 0.0 |  | 0.0 | 83.2 | 66.4 |  |  |  |  |  |  |
Restructuring charges | $ 3.0 |  |  |  | 23.0 |  |  |  |  | 0.0 |  | 7.6 | 24.3 | 1.2 |  |  |  |  |  |  |
Coal Reserves, in tons | T | 345 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclamation and Mine Shutdown Provision | $ 34.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Asset Retirement Obligations |  |  |  |  |  |  |  |  |  | 10.2 |  | 12.1 | 6.6 | 5.7 |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  | 24.3 |  | 17.9 | 30.0 | 76.4 |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  | 20.7 | $ 30.6 | 22.8 |  | 84.0 | 2.1 | 48.2 |  |  |  |  |  |  |
Springfield Reclamation Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  |  |  | 0.0 |  | 31.2 | 0.0 | 0.0 |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  |  |  |  | 31.2 |  |  |  |  |  |  |  |  |
Burton Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  |  |  |  | 0.0 |  | 52.2 | 0.0 | 0.0 |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  | $ 52.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |
Queensland’s Bowen Basin |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 37 |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  |  |  |  |  |  | 20.6 |  |  |  |  |  |  |
DTA and PBGC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage of equity method investment |  |  |  |  |  |  |  |  |  |  | 37.50% |  |  |  |  |  |  |  |  | 50.00% |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  | $ 20.5 |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  |  | $ 19.7 |  |  |  |  |  |  |  |  |  |  |
Metropolitan Collieries Pty Ltd |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Planned Divestiture, Agreed Upon Proceeds from Divestiture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200.0
New Mexico/Colorado Mining Tenement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 358.0 |  |  |  |  |
Gain (Loss) on Contract Termination |  |  |  |  |  |  |  |  |  |  |  | $ 20.0 |  |  |  |  |  |  |  |  |
Stanmore Coal Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Coal Reserves, in tons | T |  |  |  | 23 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Property, Plant, and Equipment |  |  |  | $ 22.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  | $ 20.5 |  |  |  |  | $ 20.5 |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Wambo Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of interest in joint venture |  |  |  |  |  |  |  |  |  |  |  |  | 63.7 |  |  |  |  |  |  |  |
Carrying value of net assets |  |  |  |  |  |  |  |  |  |  |  |  | 15.6 |  |  |  |  |  |  |  |
Powder River Basin Mining |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Parent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 66.50% |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.50% |  |
North Goonyella Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs Related to North Goonyella Events |  |  |  |  |  |  |  |  |  |  |  |  |  | 58.0 |  |  |  |  |  |  |
Insurance Recoveries |  |  |  |  |  |  |  |  |  |  |  |  | 125.0 |  |  |  |  |  |  |  |
Containment and idling costs |  |  |  |  |  |  |  |  |  |  |  |  | 111.5 |  |  |  |  |  |  |  |
Deductible Amount |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.0 |  |  |  |  |  |  |
North Goonyella Mine | Yancoal Technology Development Pty Ltd |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Machinery and Equipment |  |  |  |  |  |  |  |  |  |  |  |  | 54.2 |  |  |  |  |  |  |  |
North Goonyella Mine | Leased Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.7 |  |  | $ 50.7 |  |  |  |
North Goonyella Mine | Owned Equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 45.6 |  |  |  |  |  |  |
North Goonyella Mine | Longwall development, unrecoverable [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  |  |  |  |  |  |  |  |  | 39.7 |  |  |  |  |  |  |  |
North Goonyella Mine | Mine Carrying Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Costs Related to North Goonyella Events |  |  |  |  |  |  |  |  |  |  |  |  | 300.0 |  |  |  |  |  |  |  |
Kayenta Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in Restricted Cash |  |  |  |  |  |  |  |  |  |  |  |  | 53.5 |  |  |  |  |  |  |  |
Receivable for additional consideration on settlement, total |  |  |  |  | 78.5 |  |  |  |  |  |  |  | 78.5 |  |  |  | 78.5 |  |  |  |
Receivable for additional consideration on settlement |  |  |  |  | $ 35.4 |  |  |  |  |  |  |  | 35.4 |  |  |  | $ 35.4 |  |  |  |
Kayenta Mine | Scenario, Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivable for additional consideration on settlement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 16.3 |  |  |
Other | North Goonyella Mine |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OtherCommercialEvents [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision for North Goonyella equipment loss |  |  |  |  |  |  |  |  |  |  |  |  | $ 13.6 |  |  |  |  |  |  |  |

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