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Commitments and Contingencies (Details 2) AUD in Millions, $ in Millions | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 AUD | Dec. 31, 2015 USD ($) | Dec. 31, 2016 AUD | Sep. 30, 2016 AUD | Dec. 31, 2014 USD ($)
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Additional charge recorded as a result of the damages awarded to Eagle in arbitration |  |  | $ 15.6 |  |  |  |  |
Undiscounted environmental clean-up liabilities, current | $ 0.0 |  | $ 0.0 |  | $ 23.9 |  |  |
Monto Coal Pty Limited [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Ownership Percentage In Subsidiaries | 51.00% |  | 51.00% |  |  | 51.00% |  |
Loss Contingency, Damages Sought, Value | AUD |  |  |  | AUD 15.6 |  |  |  |
Loss contingency damages sought value max | AUD |  |  |  | 1,800.0 |  |  |  |
Eagle Mining [Member] |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Gain Related to Litigation Settlement |  |  |  |  | 10.8 |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Loss Contingency Accrual |  |  |  |  |  |  |  | $ 23.4
Wilkie Creek [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Loss Contingency, Damages Sought, Value | AUD |  |  |  | AUD 113.1 |  |  |  |
Loss Contingency, Loss in Period |  |  |  |  | 9.7 |  |  |
Settlement charges total | $ 9.9 |  | $ 9.9 |  |  | AUD 13.0 | AUD 13.0 |
Gold Fields [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Undiscounted environmental clean-up liabilities, total | $ 62.8 |  | $ 62.8 |  | $ 66.9 |  |  |
Prarie State Energy Campus [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Undivided Interest Percent Of New Electricity Generation Project | 5.06% |  | 5.06% |  |  | 5.06% |  |
Senior Notes [Member] |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |
Face amount of senior notes | $ 1,650.0 |  | $ 1,650.0 |  |  |  |  |
Bankruptcy unliquidated, general unsecured claim [Member] | Blue Tee [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy Claims, Amount of Claims Filed |  |  | 65.6 |  |  |  |  |
Bankruptcy additional unliquidated, in an unknown amount [Member] | Blue Tee [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy Claims, Amount of Claims Filed |  | $ 150.0 |  |  |  |  |  |
Bankruptcy unliquidated, secured and general unsecured [Member] | Gold Fields [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy Claims, Amount of Claims Filed |  |  | 2,700.0 |  |  |  |  |
Bankruptcy further contingent claims, at known and unknown sites [Member] | Blue Tee [Member] |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy Claims, Amount of Claims Filed | $ 1,200.0 |  | $ 500.0 |  |  |  |  |

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