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Segment and Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating segment results
Revenues	$ 1,313.1	$ 1,418.9	$ 1,339.3	$ 1,537.9	$ 1,684.5	$ 1,722.9	$ 1,758.0	$ 1,626.8	$ 5,609.2	$ 6,792.2	$ 7,013.7
Adjusted EBITDA									434.6	814.0	1,047.2
Additions to property, plant, equipment, and mine development									126.8	194.4	328.4
Federal coal lease expenditures									277.2	276.7	276.8
Loss from equity affiliates									15.9	107.6	40.2
Total assets	11,021.3				13,191.1				11,021.3	13,191.1	14,133.4
Property, plant, equipment and mine development, net	9,258.5				10,577.3				9,258.5	10,577.3	11,082.5
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									434.6	814.0	1,047.2
Depreciation, depletion and amortization									(572.2)	(655.7)	(740.3)
Asset retirement obligation expenses									(45.5)	(81.0)	(66.5)
Asset impairment	(377.0)		(900.8)		(154.4)				(1,277.8)	(154.4)	(528.3)
Settlement charges related to the Patriot bankruptcy reorganization									0.0	0.0	(30.6)
Amortization of basis related to equity affiliates									(4.9)	(5.7)	(6.3)
Interest expense									533.2	428.2	425.2
Interest income									7.7	15.4	15.7
Income tax benefit (provision)	(7.9)		(67.4)						176.4	(201.2)	448.3
Loss from continuing operations, net of income taxes	(497.9)	$ (144.4)	$ (1,007.2)	$ (164.4)	(478.8)	$ (154.0)	$ (72.0)	$ (44.3)	$ (1,813.9)	$ (749.1)	$ (286.0)
Revenue from external customers by geographic region
Revenue percentage									100.00%	100.00%	100.00%
U.S. [Member]
Revenue from external customers by geographic region
Revenue percentage									57.40%	59.50%	61.10%
Japan [Member]
Revenue from external customers by geographic region
Revenue percentage									8.10%	9.50%	9.80%
China [Member]
Revenue from external customers by geographic region
Revenue percentage									7.10%	6.10%	10.20%
South Korea [Member]
Revenue from external customers by geographic region
Revenue percentage									4.10%	5.20%	3.80%
Other [Member]
Revenue from external customers by geographic region
Revenue percentage									23.30%	19.70%	15.10%
Powder River Basin Mining [Member]
Operating segment results
Revenues									$ 1,865.9	$ 1,922.9	$ 1,767.3
Adjusted EBITDA									482.9	509.0	435.4
Additions to property, plant, equipment, and mine development									15.0	19.7	15.8
Federal coal lease expenditures									276.9	276.5	276.5
Loss from equity affiliates									0.0	0.0	0.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									482.9	509.0	435.4
Midwestern U.S. Mining [Member]
Operating segment results
Revenues									981.2	1,198.1	1,335.5
Adjusted EBITDA									269.7	306.9	426.0
Additions to property, plant, equipment, and mine development									51.3	57.4	27.2
Federal coal lease expenditures									0.0	0.0	0.0
Loss from equity affiliates									0.0	0.0	0.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									269.7	306.9	426.0
Asset impairment									(40.2)
Western U.S. Mining [Member]
Operating segment results
Revenues									682.3	902.8	902.3
Adjusted EBITDA									184.6	266.9	258.0
Additions to property, plant, equipment, and mine development									19.3	18.2	32.2
Federal coal lease expenditures									0.3	0.2	0.3
Loss from equity affiliates									0.0	0.0	0.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									184.6	266.9	258.0
Asset impairment										(2.7)
Australian Metallurgical Mining [Member]
Operating segment results
Revenues									1,181.9	1,613.8	1,773.4
Adjusted EBITDA									(18.2)	(151.1)	(120.0)
Additions to property, plant, equipment, and mine development									25.5	53.9	165.7
Federal coal lease expenditures									0.0	0.0	0.0
Loss from equity affiliates									0.0	0.0	0.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									(18.2)	(151.1)	(120.0)
Australian Thermal Mining [Member]
Operating segment results
Revenues									823.5	1,058.0	1,131.2
Adjusted EBITDA									193.6	264.1	270.0
Additions to property, plant, equipment, and mine development									13.6	30.2	64.6
Federal coal lease expenditures									0.0	0.0	0.0
Loss from equity affiliates									0.0	0.0	0.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									193.6	264.1	270.0
Asset impairment									(17.5)	(11.9)
Trading and Brokerage [Member]
Operating segment results
Revenues									42.8	58.4	66.0
Adjusted EBITDA									27.0	14.9	(19.9)
Additions to property, plant, equipment, and mine development									0.0	0.0	0.1
Federal coal lease expenditures									0.0	0.0	0.0
Loss from equity affiliates									0.0	0.0	0.0
Total assets	217.2				300.7				217.2	300.7	389.6
Property, plant, equipment and mine development, net	0.5				1.1				0.5	1.1	1.8
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									27.0	14.9	(19.9)
Corporate and Other [Member]
Operating segment results
Revenues									31.6	38.2	38.0
Adjusted EBITDA									(705.0)	(396.7)	(202.3)
Additions to property, plant, equipment, and mine development									2.1	15.0	22.8
Federal coal lease expenditures									0.0	0.0	0.0
Loss from equity affiliates									15.9	107.6	40.2
Total assets	1,304.0				2,167.4				1,304.0	2,167.4	2,638.2
Property, plant, equipment and mine development, net	933.9				1,332.6				933.9	1,332.6	1,479.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA									(705.0)	(396.7)	(202.3)
Asset impairment									(544.9)	(73.1)	(137.5)
U.S Mining [Member]
Operating segment results
Total assets	4,180.2				4,099.1				4,180.2	4,099.1	4,024.4
Property, plant, equipment and mine development, net	3,854.5				3,739.9				3,854.5	3,739.9	3,654.4
Australian Mining [Member]
Operating segment results
Total assets	5,319.9				6,623.9				5,319.9	6,623.9	7,081.2
Property, plant, equipment and mine development, net	$ 4,469.6				5,503.7				4,469.6	5,503.7	5,947.1
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Asset impairment									(675.2)	(66.7)	(390.8)
Middlemount Mine [Member]
Segment Reporting Information [Line Items]
Valuation Allowances and Reserves, Balance					$ 52.3					52.3
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Change in deferred tax asset valuation allowance related to equity affiliates									$ 1.0	$ (52.3)	$ 0.0