Derivatives and Fair Value Measurements - Derivatives by Balance Sheet Classification (Details) $ in Millions	Dec. 31, 2020 AUD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Designated as Hedging Instrument
Asset Derivative
Asset derivatives, gross		$ 34.6	$ 102.3
Effect of counterparty netting, derivative assets		(30.2)	(74.3)
Variation margin (held) posted on derivative assets		6.5	(22.1)
Net derivative assets and margin as classified in the balance sheets		10.9	5.9
Liability Derivative
Liability derivatives, gross		(31.9)	(74.3)
Effect of counterparty netting, derivative liabilities		30.2	74.3
Variation margin (held) posted on derivative liabilities		0.0	0.0
Net derivative liabilities and margin as classified in the balance sheets		(1.7)	0.0
Foreign currency option contracts
Liability Derivative
Derivative, Notional Amount	$ 575,000,000.0
Foreign currency option contracts \| Designated as Hedging Instrument
Asset Derivative
Asset derivatives, gross		10.3	1.1
Liability Derivative
Liability derivatives, gross		0.0	0.0
Coal contracts related to forecasted sales \| Designated as Hedging Instrument
Asset Derivative
Asset derivatives, gross		0.9	20.1
Liability Derivative
Liability derivatives, gross		(8.8)	(0.1)
Coal trading contracts \| Designated as Hedging Instrument
Asset Derivative
Asset derivatives, gross		23.4	81.1
Liability Derivative
Liability derivatives, gross		$ (23.1)	$ (74.2)

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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