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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions, $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 T	Sep. 30, 2020 $ / $	Mar. 31, 2020 AUD ($) T	Mar. 31, 2020 USD ($) T	Dec. 31, 2019 USD ($)
Derivatives, Fair Value [Line Items]
Transfers between fair value levels 1, 2, and 3	$ 0.0	$ 0.0
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount					$ 550.0
Coal
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount | T					1.6	1.6
Diesel Fuel Hedge Contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount					$ 0.0
Scenario, Forecast | Coal
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount | T			1.3
Scenario, Forecast | Minimum | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap | $ / $				0.73
Scenario, Forecast | Maximum | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap | $ / $				0.75
Coal Trading
Derivatives, Fair Value [Line Items]
Derivative, initial margin posted						$ 5.6	$ 7.9
Derivatives, amount posted in excess of initial margin requirements						2.8	1.3
Derivative, variation margin						16.8	22.1
Potential collateralization that may be requested by counterparties due to a material adverse event						$ 0.8	$ 0.0

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- Definition

Potential collateralization that may be requested by counterparties due to a material adverse event.

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- Definition

Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Cap rate on a foreign currency derivative for example, but not limited to, a cap or a collar.

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The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

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Nominal number of units used to calculate payment on derivative.

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Nominal or face amount used to calculate payment on derivative.

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Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into (out of) level 3 of the fair value hierarchy.

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Amount receivable from broker-dealers and clearing organizations, including, but not limited to, securities failed-to-deliver, certain deposits for securities borrowed, open transactions, good faith and margin deposits, commissions and floor brokerage receivables.

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