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Commitments and Contingencies - Contingencies (Details) AUD in Millions, $ in Millions |  |  |  |  | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 17, 2016 USD ($) | Oct. 13, 2016 USD ($) | Oct. 11, 2016 USD ($) | Aug. 19, 2016 USD ($) | Jun. 30, 2017 AUD defendant site | Jun. 30, 2017 USD ($) | Mar. 16, 2017 USD ($) | Dec. 31, 2016 USD ($)
Monto Coal Pty Limited |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Ownership percentage in subsidiaries |  |  |  |  |  | 51.00% |  |
Number of defendants | defendant |  |  |  |  | 3 |  |  |
Gold Fields |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Number of national priority list sites based on the Superfund Amendments and Reauthorization Act of 1986 at which Gold Fields or the former affiliate, has been named a potentially responsible party (PRP) | site |  |  |  |  | 4 |  |  |
Undiscounted environmental clean-up liabilities, total |  |  |  |  |  |  |  | $ 62.8
Minimum | Monto Coal Pty Limited |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Damages sought, value | AUD |  |  |  |  | AUD 15.6 |  |  |
Maximum | Monto Coal Pty Limited |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Damages sought, value | AUD |  |  |  |  | AUD 1,800.0 |  |  |
Berenergy Corporation vs. the Company | Berenergy |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Litigation settlement damages awarded to plaintiff |  | $ 0.9 |  |  |  |  |  |
Loss contingency accrual |  |  |  |  |  | $ 13.1 |  |
Bankruptcy unliquidated, general unsecured claim | Blue Tee |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy claims, amount of claims filed |  |  |  | $ 65.6 |  |  |  |
Bankruptcy additional unliquidated, in an unknown amount | Blue Tee |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy claims, amount of claims filed |  |  |  | 150.0 |  |  |  |
Bankruptcy further contingent claims, at known and unknown sites | Blue Tee |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy claims, amount of claims filed |  |  |  | $ 500.0 |  |  |  |
Bankruptcy unliquidated, secured and general unsecured | Gold Fields |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy claims, amount of claims filed | $ 1,200.0 |  | $ 2,700.0 |  |  |  |  |
Debtors vs. Blue Tee |  |  |  |  |  |  |  |
Loss Contingency [Abstract] |  |  |  |  |  |  |  |
Bankruptcy claims, amount of federal tax refunds to be foregone to settle claims |  |  |  |  |  |  | $ 11.0 |
Bankruptcy claims, amount of insurance settlement proceeds to be transferred to settle claims |  |  |  |  |  |  | 12.0 |
Bankruptcy claims, amount to be paid directly to settle claims |  |  |  |  |  |  | $ 20.0 |

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