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Pension and Postretirement Benefit Costs - Narrative (Details) | Jun. 30, 2021
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Defined benefit pension plans |
Defined Benefit Plan Disclosure [Line Items] |
Funding threshold | 80.00%

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- 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

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- 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

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