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Amortization of basis difference associated with the step-up of equity method investments as part of the acquisition of Macarthur Coal Limited.

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Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization.

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Segment Reporting, Revenue Percentage

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Settlement charges related to the Patriot bankruptcy reorganization

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating segment results
Revenues	$ 1,684.5us-gaap_Revenues	$ 1,722.9us-gaap_Revenues	$ 1,758.0us-gaap_Revenues	$ 1,626.8us-gaap_Revenues	$ 1,742.8us-gaap_Revenues	$ 1,797.6us-gaap_Revenues	$ 1,725.3us-gaap_Revenues	$ 1,748.0us-gaap_Revenues	$ 6,792.2us-gaap_Revenues	$ 7,013.7us-gaap_Revenues	$ 8,077.5us-gaap_Revenues
Adjusted EBITDA									814.0	1,047.2	1,836.5
Total assets	13,191.1us-gaap_Assets				14,133.4us-gaap_Assets				13,191.1us-gaap_Assets	14,133.4us-gaap_Assets	15,809.0us-gaap_Assets
Property, plant, equipment and mine development, net	10,577.3us-gaap_PropertyPlantAndEquipmentNet				11,082.5us-gaap_PropertyPlantAndEquipmentNet				10,577.3us-gaap_PropertyPlantAndEquipmentNet	11,082.5us-gaap_PropertyPlantAndEquipmentNet	11,801.7us-gaap_PropertyPlantAndEquipmentNet
Additions to property, plant, equipment, and mine development									194.4	328.4	986.0
Federal coal lease expenditures									276.7btu_FederalCoalLeaseExpenditure	276.8btu_FederalCoalLeaseExpenditure	276.5btu_FederalCoalLeaseExpenditure
Loss from equity affiliates									107.6	40.2	61.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									814.0	1,047.2	1,836.5
Depreciation, depletion and amortization									(655.7)	(740.3)	(663.4)
Asset retirement obligation expenses									(81.0)	(66.5)	(67.0)
Asset impairment and mine closure costs	(154.4)				(506.8)		(21.5)		(154.4)	(528.3)	(929.0)
Settlement charges related to the Patriot bankruptcy reorganization					(30.6)				0	(30.6)	0
Amortization of basis related to equity affiliates									(5.7)	(6.3)	(4.6)
Interest expense									(428.2)us-gaap_InterestExpense	(425.2)us-gaap_InterestExpense	(405.6)us-gaap_InterestExpense
Interest income									15.4us-gaap_InvestmentIncomeInterest	15.7us-gaap_InvestmentIncomeInterest	24.5us-gaap_InvestmentIncomeInterest
Income tax (provision) benefit									(201.2)us-gaap_IncomeTaxExpenseBenefit	448.3us-gaap_IncomeTaxExpenseBenefit	(262.3)us-gaap_IncomeTaxExpenseBenefit
Loss from continuing operations, net of income taxes	(478.8)	(154.0)	(72.0)	(44.3)	(401.1)	24.0	101.4	(10.3)	(749.1)	(286.0)	(470.9)
Revenue from external customers by geographic region
Revenue percentage									100.00%btu_SegmentReportingRevenuePercentage	100.00%btu_SegmentReportingRevenuePercentage	100.00%btu_SegmentReportingRevenuePercentage
U.S. [Member]
Revenue from external customers by geographic region
Revenue percentage									59.50%	61.10%	55.40%
Japan [Member]
Revenue from external customers by geographic region
Revenue percentage									9.50%	9.80%	12.30%
China [Member]
Revenue from external customers by geographic region
Revenue percentage									6.10%	10.20%	6.80%
South Korea [Member]
Revenue from external customers by geographic region
Revenue percentage									5.20%	3.80%	5.80%
Other [Member]
Revenue from external customers by geographic region
Revenue percentage									19.70%	15.10%	19.70%
Western U.S. Mining [Member]
Operating segment results
Revenues									2,825.7	2,669.6	2,949.3
Adjusted EBITDA									770.4	698.3	832.8
Total assets	3,431.3				3,307.6				3,431.3	3,307.6	3,257.9
Property, plant, equipment and mine development, net	3,127.8				2,986.8				3,127.8	2,986.8	2,896.1
Additions to property, plant, equipment, and mine development									37.9	48.0	121.4
Federal coal lease expenditures									276.7	276.8	276.5
Loss from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									770.4	698.3	832.8
Asset impairment and mine closure costs									(2.7)		(2.4)
Midwestern U.S. Mining [Member]
Operating segment results
Revenues									1,198.1	1,335.5	1,403.7
Adjusted EBITDA									301.4	428.3	427.0
Total assets	667.8				716.8				667.8	716.8	728.5
Property, plant, equipment and mine development, net	612.1				667.6				612.1	667.6	707.5
Additions to property, plant, equipment, and mine development									57.4	27.2	74.7
Federal coal lease expenditures									0	0	0
Loss from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									301.4	428.3	427.0
Asset impairment and mine closure costs											(45.0)
Australian Mining [Member]
Operating segment results
Revenues									2,671.8	2,904.6	3,503.6
Adjusted EBITDA									74.4	316.6	938.9
Total assets	6,623.9				7,081.2				6,623.9	7,081.2	7,990.8
Property, plant, equipment and mine development, net	5,503.7				5,947.1				5,503.7	5,947.1	6,595.5
Additions to property, plant, equipment, and mine development									84.1	230.3	743.4
Federal coal lease expenditures									0	0	0
Loss from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									74.4	316.6	938.9
Asset impairment and mine closure costs									(78.6)	(390.8)	(806.7)
Trading and Brokerage [Member]
Operating segment results
Revenues									58.4	66.0	199.9
Adjusted EBITDA									14.9	(19.9)	119.7
Total assets	300.7				389.6				300.7	389.6	543.9
Property, plant, equipment and mine development, net	1.1				1.8				1.1	1.8	1.6
Additions to property, plant, equipment, and mine development									0	0.1	0.1
Federal coal lease expenditures									0	0	0
Loss from equity affiliates									0	0	0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									14.9	(19.9)	119.7
Corporate and Other [Member]
Operating segment results
Revenues									38.2	38.0	21.0
Adjusted EBITDA									(347.1)	(376.1)	(481.9)
Total assets	2,167.4				2,638.2				2,167.4	2,638.2	3,287.9
Property, plant, equipment and mine development, net	1,332.6				1,479.2				1,332.6	1,479.2	1,601.0
Additions to property, plant, equipment, and mine development									15.0	22.8	46.4
Federal coal lease expenditures									0	0	0
Loss from equity affiliates									107.6	40.2	61.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									(347.1)	(376.1)	(481.9)
Asset impairment and mine closure costs									(73.1)	(137.5)	(74.9)
Middlemount Mine [Member]
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Change in valuation allowance against loss carryforwards									$ (52.3)	$ 0	$ 0