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Pension and Postretirement Benefit Costs - Narrative (Details)	Mar. 31, 2021
Defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Funding threshold	80.00%

X

- Definition

Percentage of company's qualified pension plans on a GAAP accounting basis.

+ References

No definition available.

+ Details

Name:	btu_PercentageOfCompanysQualifiedPensionPlansOnGaapAccountingBasis
Namespace Prefix:	btu_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

X

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

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: