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Commitments and Contingencies - Contingencies (Details) $ in Millions | 3 Months Ended | 21 Months Ended | 93 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 AUD ($) | Sep. 30, 2019 AUD ($) | Sep. 30, 2019 AUD ($) | Oct. 24, 2017 lawsuits | Sep. 15, 2017 coalleases
Loss Contingencies [Line Items] |  |  |  |  |
Number of Lawsuits | lawsuits |  |  |  | 3 |
Monto Coal Pty Limited |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Legal Fees | $ |  | $ 29 | $ 48 |  |
Maximum | Monto Coal Pty Limited |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss Contingency, Damages Sought, Value | $ | $ 1,100 |  |  |  |
Powder River Basin Mining |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Number of Leases | coalleases |  |  |  |  | 4
North Antelope Rochelle Mine |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Number of Leases | coalleases |  |  |  |  | 2

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