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Commitments and Contingencies - Contingencies (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 AUD ($) | Oct. 24, 2017 lawsuits
Loss Contingencies [Line Items] |  |
Number of Lawsuits | lawsuits |  | 3
Monto Coal Pty Limited |  |
Loss Contingencies [Line Items] |  |
LossContingencyDamagesSoughtValueMax | $ 18.0 |
Maximum | Monto Coal Pty Limited |  |
Loss Contingencies [Line Items] |  |
Loss Contingency, Damages Sought, Value | $ 1,100.0 |

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