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Commitments and Contingencies - Contingencies (Details) - 9 months ended Sep. 30, 2018 $ in Millions, $ in Billions | AUD ($) | USD ($)
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Maximum | Monto Coal Pty Limited |  |
Loss Contingency [Abstract] |  |
Damages sought, value | $ 1.8 |
Berenergy Corporation vs. the Company | Berenergy |  |
Loss Contingency [Abstract] |  |
Loss contingency accrual |  | $ 13.1
Reasonable Value of Oil Reserves |  | $ 1.0

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