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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) T	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) T	Jun. 30, 2018 USD ($)	Mar. 31, 2020 $ / $	Dec. 31, 2019 T	Jun. 30, 2019 AUD ($) T	Dec. 31, 2018 USD ($)
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							$ 1,000.0
Coal
Derivatives, Fair Value [Line Items]
Derivative, nonmonetary notional amount | T	3.2		3.2				3.2
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						1.7
Scenario, Forecast | Minimum | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap | $ / $					0.74
Scenario, Forecast | Maximum | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap | $ / $					0.77
Coal Trading
Derivatives, Fair Value [Line Items]
Derivative, initial margin posted	$ 15,300,000		$ 15,300,000					$ 16,700,000
Derivatives, amount posted in excess of initial margin requirements	300,000		300,000					2,200,000
Derivative, variation margin	46,100,000		46,100,000					21,800,000
Potential collateralization that may be requested by counterparties due to a material adverse event	$ 0		$ 0					$ 1,300,000
Coal Trading Positions [Member] | External Credit Rating, Investment Grade [Member] | Coal Trading | Credit Concentration Risk [Member]
Derivatives, Fair Value [Line Items]
Credit exposure related to Coal Trading activities	62.00%		62.00%				62.00%
Coal Trading Positions [Member] | Non Rated [Member] | Coal Trading | Credit Concentration Risk [Member]
Derivatives, Fair Value [Line Items]
Credit exposure related to Coal Trading activities	38.00%		38.00%				38.00%

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- Definition

Credit Exposure Related to Coal Trading Activities

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

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- Definition

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313

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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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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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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-Topic 820
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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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