Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2016		Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Gain on VEBA settlement				$ 68.1		$ 0.0
Total Assets	$ 12,235.2		$ 12,235.2	12,235.2			$ 10,946.9
Reportable segment results
Revenues	1,207.1	$ 1,418.9	2,123.9	3,274.5		4,296.1
Adjusted EBITDA	108.3	129.0		188.9		381.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Depreciation, depletion and amortization	(117.8)	(136.0)	(216.8)	(345.5)		(430.6)
Asset impairment	0.0	0.0		(17.2)		(900.8)
Asset retirement obligation expenses	(12.7)	(12.3)	(22.4)	(37.3)		(40.4)
Selling and Administrative expenses related to debt restructuring	0.0	0.0		(21.5)		0.0
Amortization of basis difference related to equity affiliates	0.0	(0.7)		0.0		(4.2)
Interest expense	(58.5)	(118.5)	(95.5)	(243.7)		(344.0)
Loss on early debt extinguishment	0.0	0.0		0.0		(67.8)
Interest income	1.3	1.4	2.3	4.0		6.6
Reorganization Items, net	(29.7)	0.0	125.1	(125.1)		0.0
Income tax (benefit) provision	12.9	(6.9)	30.9	108.7		83.2
Loss from continuing operations, net of income taxes	(95.6)	(144.4)	$ (253.9)	(488.1)		(1,316.0)
Powder River Basin Mining [Member]
Reportable segment results
Revenues	419.6	475.0		1,062.2		1,393.9
Adjusted EBITDA	123.9	120.0		278.3		354.7
Midwestern U.S. Mining [Member]
Reportable segment results
Revenues	211.0	249.4		599.6		768.2
Adjusted EBITDA	59.1	72.3		172.4		218.6
Western U.S. Mining [Member]
Reportable segment results
Revenues	162.4	177.3		387.0		526.9
Adjusted EBITDA	34.3	45.7		83.2		147.7
Australian Metallurgical Mining [Member]
Reportable segment results
Revenues	232.5	280.4		682.8		924.6
Adjusted EBITDA	(34.5)	(15.4)		(121.0)		(2.2)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Asset impairment						(527.0)
Australian Thermal Mining [Member]
Reportable segment results
Revenues	197.9	205.3		561.4		615.2
Adjusted EBITDA	48.9	49.4		137.2		153.9
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Asset impairment						(8.2)
Trading and Brokerage [Member]
Reportable segment results
Revenues	(19.2)	24.5		(32.6)		42.7
Adjusted EBITDA	(31.3)	29.4		(90.4)		30.4
Corporate and Other [Member]
Reportable segment results
Revenues	2.9	7.0		14.1		24.6
Adjusted EBITDA	(92.1)	(172.4)		(270.8)	[1]	(521.5)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Asset impairment						(365.6)
Middlemount Mine [Member]
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Change in deferred tax asset valuation allowance related to equity affiliates	$ 0.6	$ (0.4)		$ 0.6		$ 0.4

[1] (1) Includes a gain of $68.1 million during the nine months ended September 30, 2016 related to the 2016 Settlement Agreement described in Note 20 "Matters related to the Bankruptcy of Patriot Coal Corporation"