Summary Quarterly Financial Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended																6 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		Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of the unaudited quarterly results of operations
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
Operating profit (loss)	(470.8)		(20.4)		(975.8)		2.2		(207.0)		36.2		32.8		2.9			(1,464.8)	(135.1)	(324.8)
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)
Net loss	(470.2)		(301.9)		(1,043.5)		(173.3)		(513.0)		(149.0)		(71.2)		(44.1)			(1,988.9)	(777.3)	(512.6)
Net loss attributable to common stockholders	$ (469.4)		$ (304.7)		$ (1,045.3)		$ (176.6)		$ (514.6)		$ (150.6)		$ (73.3)		$ (48.5)			$ (1,996.0)	$ (787.0)	$ (524.9)
Basic and diluted EPS — continuing operations(1)	$ (27.28)	[1]	$ (8.08)	[1]	$ (55.59)	[1]	$ (9.31)	[1]	$ (26.88)	[2]	$ (8.72)	[2]	$ (4.16)	[2]	$ (2.74)	[2]		$ (100.34)	$ (42.52)	$ (16.80)
Diluted EPS — continuing operations(1)																		$ (100.34)	$ (42.52)	$ (16.80)
Weighted average shares used in calculating basic and diluted EPS	18.2		18.2		18.2		18.0		17.9		17.9		17.9		17.9			18.1	17.9	17.8
Weighted average shares used in calculating diluted EPS																		18.1	17.9	17.8
Summary Quarterly Financial Information (Textuals) [Abstract]
Asset impairment	$ 377.0				$ 900.8				$ 154.4									$ 1,277.8	$ 154.4	$ 528.3
Restructuring and pension settlement charges					21.2				26.0									23.5	26.0	11.9
Loss on early debt extinguishment					(8.3)		$ (59.5)											(67.8)	(1.6)	(16.9)
Loss from discontinued operations, net of income taxes			$ 155.1				7.6											(175.0)	(28.2)	(226.6)
Additional charge recorded as a result of the damages awarded to Eagle in arbitration											$ 10.6				$ 15.6			(15.6)
Remeasurement of foreign taxes	(0.5)		$ 0.8				$ 0.2		(1.2)		1.2		$ 1.3		1.4			0.5	2.7	44.3
Income tax benefit related to asset impairment	7.9				67.4													(176.4)	201.2	(448.3)
Change in fair value of credit support provided to Patriot					$ 34.7				34.1
Australia Deferred Tax Assets [Member]
Summary Quarterly Financial Information (Textuals) [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase, Amount									90.4		80.6		75.7		$ 42.6			(101.3)
Australian Mining [Member]
Summary Quarterly Financial Information (Textuals) [Abstract]
Asset impairment																		675.2	66.7	$ 390.8
US Deferred Tax Assets [Member]
Summary Quarterly Financial Information (Textuals) [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase, Amount	$ 177.0								280.1									(177.0)
Income Tax Expense, Continuing Operations, Adjustment of Deferred Tax Asset																		182.7
Australian Minerals and Resource Rent Tax [Member]
Summary Quarterly Financial Information (Textuals) [Abstract]
Income Tax Expense, Continuing Operations, Adjustment of Deferred Tax Asset											$ 70.1						$ 54.0
Junior Subordinated Debt [Member]
Summary Quarterly Financial Information (Textuals) [Abstract]
Refinancing charges classified as interest expense													1.6					$ 1.6
Western U.S. Mining Segment Customer [Member] | Positive Outcome of Litigation [Member]
Summary Quarterly Financial Information (Textuals) [Abstract]
Contracts Revenue													$ 43.2						$ 33.5
Interest In Middlemount Coal Pty Limited [Member]
Summary Quarterly Financial Information (Textuals) [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase, Amount									$ 52.3

[1] EPS for the quarters may not sum to the amounts for the year as each period is computed on a discrete basis.

[2] EPS for the quarters may not sum to the amounts for the year as each period is computed on a discrete basis