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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions, $ in Millions, $ in Millions	Dec. 31, 2019 T	Sep. 30, 2019 $ / $	Mar. 31, 2019 AUD ($) T	Mar. 31, 2019 USD ($) T	Dec. 31, 2018 USD ($)
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount			$ 975.0
Coal
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount | T			3.7	3.7
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	2.2
Scenario, Forecast | Minimum | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Exchange Rate Cap | $ / $		0.76
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, Fair Value, Amount Offset Against Collateral, Net				$ 18.2	$ 21.8
Derivative, Collateral, Right to Reclaim Cash				15.5	16.7
Receivables from Brokers-Dealers and Clearing Organizations				6.0	2.2
Potential collateralization that may be requested by counterparties due to a material adverse event				$ 0.1	$ 1.3
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			43.00%	43.00%
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			57.00%	57.00%

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- Definition

Credit Exposure Related to Coal Trading Activities

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- Definition

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

+ References

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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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- Definition

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
-Sentence Column B

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226016-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=35710130&loc=d3e41645-110958

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 310
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-Section 45
-Paragraph 2
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