Commitments and Contingencies (Details) $ in Millions, $ in Millions	12 Months Ended		336 Months Ended
	Dec. 31, 2019 AUD ($)	Dec. 31, 2018 AUD ($)	Dec. 31, 2042	Dec. 31, 2019 USD ($)
Long-term Purchase Commitment [Line Items]
Take-or-pay obligations				$ 1,100.0
Take-or-pay Obligations Due In One Year				116.0
Capital Additions
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months				$ 39.7
Monto Coal Pty Limited
Long-term Purchase Commitment [Line Items]
LossContingencyDamagesSoughtValueMax	$ 18.0	$ 1,100.0
Scenario, Forecast
Long-term Purchase Commitment [Line Items]
Take-or-pay Arrangement Terms Years (Maximum)			23 years