The Company elected to exercise the 30-day grace period with respect to its 6.50% and 10.00% Senior Notes.

No definition available.

Name:	btu_A30daygraceperiod
Namespace Prefix:	btu_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount represents the maximum face amount of all letters of credit issued under the L/C Facility and the Bonding L/C Facility without DIP Lender consent.

+ References

No definition available.

+ Details

Name:	btu_AggregateLettersofCreditMaximum
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Applicable Rate, Eurocurrency Rate Loan

+ References

No definition available.

+ Details

Name:	btu_ApplicableInterestRateEurocurrencyRateLoan
Namespace Prefix:	btu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Applicable Rate, Base Rate Loan

+ References

No definition available.

+ Details

Name:	btu_ApplicableRateBaseRateLoan
Namespace Prefix:	btu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest expense

+ References

No definition available.

+ Details

Name:	btu_Contractualinterestexpense
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Debt Instrument, Libor Interest Rate

+ References

No definition available.

+ Details

Name:	btu_DebtInstrumentLiborInterestRate
Namespace Prefix:	btu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Outstanding Principal

+ References

No definition available.

+ Details

Name:	btu_DebtInstrumentOutstandingPrincipal
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate principal amount of the 2016 Senior notes that were redeemed by the Company.

+ References

No definition available.

+ Details

Name:	btu_DebtInstrumentRedemption2016Notes
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Extension Fee

+ References

No definition available.

+ Details

Name:	btu_ExtensionFee
Namespace Prefix:	btu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Intercompany Loan, Potential Increase

+ References

No definition available.

+ Details

Name:	btu_IntercompanyLoanPotentialIncrease
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Intercompany Loans

+ References

No definition available.

+ Details

Name:	btu_IntercompanyLoans
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Intercompany Loans, Outstanding

+ References

No definition available.

+ Details

Name:	btu_IntercompanyLoansOutstanding
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Interest Expense, Adequate Protection Payments

+ References

No definition available.

+ Details

Name:	btu_InterestExpenseAdequateProtectionPayments
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest Expense, Stayed Amount

+ References

No definition available.

+ Details

Name:	btu_InterestExpenseStayedAmount
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Letters of Credit, Additional

+ References

No definition available.

+ Details

Name:	btu_LettersofCreditAdditional
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lien Value of Assets Collateralizing the Intercompany Loan Facility

+ References

No definition available.

+ Details

Name:	btu_LienValueofAssetsCollateralizingtheIntercompanyLoanFacility
Namespace Prefix:	btu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount represents the tender offer premiums paid on the 2016 Senior Notes repurchase.

+ References

No definition available.

+ Details

Name:	btu_TenderOfferPremiumsPaidon2016SeniorNotesRepurchase
Namespace Prefix:	btu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

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

- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

+ Details

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

- Definition

Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).

+ References

No definition available.

+ Details

Name:	us-gaap_DebtIssuanceCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12317-112629

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12355-112629

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.9)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_InterestExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate carrying amount, as of the balance sheet date, of investments and other noncurrent assets not separately disclosed in the balance sheet due to materiality considerations. Noncurrent assets are expected to be realized or consumed after one year (or the normal operating cycle, if longer).

+ References

No definition available.

+ Details

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

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20, 22
-Article 5

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

Name:	us-gaap_WriteOffOfDeferredDebtIssuanceCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_TermLoanFacilityBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_A6.00SeniorNotesdueNovember2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_A6.50SeniorNotesdueSeptember2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_A6.25SeniorNotesdueNovember2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_A10.00SeniorSecuredSecondLienNotesDue2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_A7.875SeniorNotesdueNovember2026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_JuniorSubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_A7.375SeniorNotesdueNovember2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_DIPFacilityTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_DIPFacilityInitiallyAvailableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_DIPFacilityRemainingAvailabilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_DIPTermLoanFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_IntercompanyLoanFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_DIPFacilityLetterofCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_DIPFacilityBondingAccommodationFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=btu_DIPCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-term Debt Textuals (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
Long-term Debt Textuals (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	May 18, 2016	Apr. 18, 2016	Apr. 15, 2016	Apr. 13, 2016	Mar. 31, 2016	Dec. 31, 2015	Apr. 15, 2015
Debt Instrument [Line Items]
Posted but undrawn letters of credit	$ 144.9		$ 144.9	$ 144.9
Contractual interest expense	151.4			$ 417.1
30-day grace period				30-day grace period
Interest expense	58.5	$ 118.5	95.5	$ 243.7	$ 344.0
Interest Expense, Adequate Protection Payments	39.6			64.2
Interest Expense, Stayed Amount	92.9			173.4
Loss on early debt extinguishment	0.0	0.0		0.0	(67.8)
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	1,388.4		1,388.4	1,388.4						$ 947.0	$ 0.0
Credit agreement borrowing capacity	1,650.0		1,650.0	1,650.0
Posted but undrawn letters of credit	675.0		675.0	675.0					$ 675.0
Letters of Credit, Additional	441.4		441.4	441.4
Collateral in support of letters of credit	343.7		343.7	343.7
2013 Term Loan Facility due September 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	1,154.5		1,154.5	1,154.5							1,156.3
Debt Instrument, Outstanding Principal	1,170.0		1,170.0	1,170.0
6.00% Senior Notes due November 2018 [Member]
Debt Instrument [Line Items]
Long-term debt	1,509.9		1,509.9	1,509.9							1,508.9
Debt Instrument, Outstanding Principal	$ 1,518.8		$ 1,518.8	$ 1,518.8
Stated interest rate - percentage	6.00%		6.00%	6.00%
6.50% Senior Notes due September 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 645.8		$ 645.8	$ 645.8							645.5
Debt Instrument, Outstanding Principal	$ 650.0		$ 650.0	$ 650.0
Stated interest rate - percentage	6.50%		6.50%	6.50%
Interest expense				$ 21.1
6.25% Senior Notes due November 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,327.7		$ 1,327.7	1,327.7							1,327.0
Debt Instrument, Outstanding Principal	$ 1,339.6		$ 1,339.6	$ 1,339.6
Stated interest rate - percentage	6.25%		6.25%	6.25%
10.00% Senior Secured Second Lien Notes Due March 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 962.3		$ 962.3	$ 962.3							960.4
Debt Instrument, Outstanding Principal	$ 1,000.0		$ 1,000.0	$ 1,000.0
Stated interest rate - percentage	10.00%		10.00%	10.00%
Interest expense				$ 50.0
Debt Instrument, Face Amount		1,000.0			1,000.0
7.875% Senior Notes due November 2026 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 245.9		$ 245.9	245.9							245.8
Debt Instrument, Outstanding Principal	$ 250.0		$ 250.0	$ 250.0
Stated interest rate - percentage	7.875%		7.875%	7.875%
Convertible Junior Subordinated Debentures due December 2066 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 367.1		$ 367.1	$ 367.1							366.3
Debt Instrument, Outstanding Principal	$ 732.5		$ 732.5	$ 732.5
7.375% Senior Notes due November 2016 [Member]
Debt Instrument [Line Items]
Stated interest rate - percentage	7.375%		7.375%	7.375%
Debt Instrument, Repurchased Face Amount		650.0			650.0
Debt Instrument, Repurchase Amount		$ 566.9			566.9
Debt Instrument Redemption 2016 Notes												$ 83.1
Loss on early debt extinguishment					(67.8)
Tender Offer Premiums Paid on 2016 Senior Notes Repurchase					66.4
Write off of Deferred Debt Issuance Cost					$ 1.4
DIP Facility - Term Loan [Member]
Debt Instrument [Line Items]
Long-term debt									500.0
DIP Facility - Initially Available [Member]
Debt Instrument [Line Items]
Long-term debt							$ 200.0	$ 200.0	200.0
DIP Facility - Remaining Availability [Member]
Debt Instrument [Line Items]
Long-term debt						$ 300.0			300.0
DIP Term Loan Facility [Member]
Debt Instrument [Line Items]
Long-term debt	$ 470.5		$ 470.5	$ 470.5							$ 0.0
Applicable Interest Rate, Eurocurrency Rate Loans	9.00%		9.00%	9.00%
Debt Instrument, Libor Interest Rate	1.00%		1.00%	1.00%
Applicable Rate, Base Rate Loan	8.00%		8.00%	8.00%
Extension Fee	2.50%		2.50%	2.50%
Debt Issuance Cost	$ 25.6
Intercompany Loan Facility [Member]
Debt Instrument [Line Items]
Intercompany Loans	250.0		$ 250.0	$ 250.0
Intercompany Loan, Potential Increase	$ 200.0		$ 200.0	$ 200.0
Lien Value Threshold of Assets Collateralizing the Intercompany Loan Facility	50.00%		50.00%	50.00%
Intercompany Loans, Outstanding	$ 30.0		$ 30.0	$ 30.0
DIP Facility - Letter of Credit [Member]
Debt Instrument [Line Items]
Posted but undrawn letters of credit								100.0	100.0
DIP Facility - Bonding Accommodation Facility [Member]
Debt Instrument [Line Items]
Long-term debt								$ 200.0	200.0
DIP Credit Agreement [Member]
Debt Instrument [Line Items]
Long-term debt									800.0
Aggregate Letters of Credit, Maximum									$ 50.0