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