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Segment and Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended										9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating segment results
Revenues	$ 1,440.8	$ 1,207.1	$ 1,040.2	$ 1,027.2	$ 1,313.1			$ 1,418.9	$ 1,339.3	$ 1,537.9	$ 3,564.7	$ 4,715.3	$ 5,609.2	$ 6,792.2
Adjusted EBITDA												492.2	434.6	814.0
Additions to property, plant, equipment, and mine development											106.7	126.6	126.8	194.4
Federal coal lease expenditures												(249.0)	277.2	276.7
Loss from equity affiliates	(28.8)										26.2	(16.2)	15.9	107.6
Total assets	11,777.7				10,946.9						11,777.7	11,777.7	10,946.9	13,191.1
Property, plant, equipment and mine development, net	8,776.7				9,258.5						8,776.7	8,776.7	9,258.5	10,577.3
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA												492.2	434.6	814.0
Depreciation, depletion and amortization											(336.7)	(465.4)	(572.2)	(655.7)
Asset retirement obligation expenses											(26.9)	(41.8)	(45.5)	(81.0)
Selling and Administrative expenses related to debt restructuring												21.5	0.0	0.0
Asset impairment	(230.7)			(17.2)	(377.0)	$ (900.8)					(230.7)	(247.9)	(1,277.8)	(154.4)
Amortization of basis related to equity affiliates												0.0	4.9	5.7
Interest expense												(298.6)	465.4	(426.6)
Loss on early debt extinguishment	(29.5)					8.3	$ 59.5				(29.5)	29.5	67.8	1.6
Interest income											4.0	5.7	7.7	15.4
Reorganization Items	(33.9)	(29.7)	(95.4)								(159.0)	(159.0)	0.0	0.0
Income tax benefit (provision)					(7.9)	$ (67.4)					22.7	94.5	207.1	(147.4)
Loss from continuing operations, net of income taxes	(175.2)	$ (97.7)	$ (223.2)	$ (167.7)	(502.0)			$ (126.4)	$ (1,006.0)	$ (148.8)	(423.6)	$ (663.8)	$ (1,783.2)	$ (695.3)
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												54.70%	57.40%	59.50%
Japan [Member]
Revenue from external customers by geographic region
Revenue percentage												6.90%	8.10%	9.50%
China [Member]
Revenue from external customers by geographic region
Revenue percentage												5.40%	7.10%	6.10%
South Korea [Member]
Revenue from external customers by geographic region
Revenue percentage												1.50%	4.10%	5.20%
Other [Member]
Revenue from external customers by geographic region
Revenue percentage												31.50%	23.30%	19.70%
Powder River Basin Mining [Member]
Operating segment results
Revenues												$ 1,473.3	$ 1,865.9	$ 1,922.9
Adjusted EBITDA												379.9	482.9	509.0
Additions to property, plant, equipment, and mine development												33.0	15.0	19.7
Federal coal lease expenditures												248.4	276.9	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												379.9	482.9	509.0
Midwestern U.S. Mining [Member]
Operating segment results
Revenues												792.5	981.2	1,198.1
Adjusted EBITDA												217.3	269.7	306.9
Additions to property, plant, equipment, and mine development												18.7	51.3	57.4
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												217.3	269.7	306.9
Asset impairment													(40.2)
Western U.S. Mining [Member]
Operating segment results
Revenues												526.0	682.3	902.8
Adjusted EBITDA												101.6	184.6	266.9
Additions to property, plant, equipment, and mine development												20.8	19.3	18.2
Federal coal lease expenditures												0.6	0.3	0.2
Loss from equity affiliates												0.0	0.0	0.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA												101.6	184.6	266.9
Asset impairment														(2.7)
Australian Metallurgical Mining [Member]
Operating segment results
Revenues												1,090.4	1,181.9	1,613.8
Adjusted EBITDA												(16.3)	(18.2)	(151.1)
Additions to property, plant, equipment, and mine development												29.9	25.5	53.9
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												(16.3)	(18.2)	(151.1)
Australian Thermal Mining [Member]
Operating segment results
Revenues												824.9	823.5	1,058.0
Adjusted EBITDA												217.6	193.6	264.1
Additions to property, plant, equipment, and mine development												22.1	13.6	30.2
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												217.6	193.6	264.1
Asset impairment													(17.5)	(11.9)
Trading and Brokerage [Member]
Operating segment results
Revenues												(10.9)	42.8	58.4
Adjusted EBITDA												(72.2)	27.0	14.9
Additions to property, plant, equipment, and mine development												0.0	0.0	0.0
Federal coal lease expenditures												0.0	0.0	0.0
Loss from equity affiliates												0.0	0.0	0.0
Total assets	128.7				217.2						128.7	128.7	217.2	300.7
Property, plant, equipment and mine development, net	0.2				0.5						0.2	0.2	0.5	1.1
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA												(72.2)	27.0	14.9
Corporate and Other [Member]
Operating segment results
Revenues												19.1	31.6	38.2
Adjusted EBITDA												(335.7)	(705.0)	(396.7)
Additions to property, plant, equipment, and mine development												2.1	2.1	15.0
Federal coal lease expenditures												0.0	0.0	0.0
Loss from equity affiliates												(16.2)	15.9	107.6
Total assets	1,990.9				1,304.0						1,990.9	1,990.9	1,304.0	2,167.4
Property, plant, equipment and mine development, net	900.1				933.9						900.1	900.1	933.9	1,332.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Adjusted EBITDA												(335.7)	(705.0)	(396.7)
Asset impairment													(544.9)	(73.1)
U.S Mining [Member]
Operating segment results
Total assets	4,255.9				4,105.8						4,255.9	4,255.9	4,105.8	4,099.1
Property, plant, equipment and mine development, net	3,970.6				3,854.5						3,970.6	3,970.6	3,854.5	3,739.9
Australian Mining [Member]
Operating segment results
Total assets	5,402.2				5,319.9						5,402.2	5,402.2	5,319.9	6,623.9
Property, plant, equipment and mine development, net	$ 3,905.8				$ 4,469.6						$ 3,905.8	3,905.8	4,469.6	5,503.7
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Asset impairment													(675.2)	(66.7)
Middlemount Mine [Member]
Segment Reporting Information [Line Items]
Valuation Allowances and Reserves, Balance														52.3
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations [Abstract]
Change in deferred tax asset valuation allowance related to equity affiliates												$ (7.5)	$ (1.0)	$ 52.3