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

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

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226038-175313

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=d3e41678-113959

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-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

+ Details

Name:	us-gaap_DerivativeCollateralRightToReclaimCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeExchangeRateCap1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeExchangeRateFloor1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Nominal mass used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountMass
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:massItemType
Balance Type:	na
Period Type:	duration

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativesFairValueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CoalContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=btu_DieselFuelHedgeContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TradingActivityByTypeAxis=btu_CoalTradingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivatives and Fair Value Measurements - Narrative (Details) $ in Millions	6 Months Ended	9 Months Ended
	Jun. 30, 2021 AUD ($) t	Sep. 30, 2021 t	Dec. 31, 2021 $ / $	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Notional amount	$ 580,000,000.0
Coal trading contracts
Derivatives, Fair Value [Line Items]
Derivative notional amount (in tonnes) \| t	2,800,000
Diesel Fuel Hedge Contracts
Derivatives, Fair Value [Line Items]
Notional amount	$ 0
Forecast \| Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate floor (in dollars per share) \| $ / $			0.80
Derivative, exchange rate cap (in dollars per share) \| $ / $			0.81
Forecast \| Coal trading contracts
Derivatives, Fair Value [Line Items]
Derivative notional amount (in tonnes) \| t		400,000
Coal Trading
Derivatives, Fair Value [Line Items]
Derivative, initial margin posted				$ 14.4	$ 3.0