Resource Management and Other Commercial Events (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Resource Management and Other Commercial Events (Textuals) [Abstract]
Revenues	$ 2,016.9	$ 2,058.8	$ 1,981.1	$ 2,020.7	$ 2,229.8	$ 1,980.6	$ 1,961.7	$ 1,723.8	$ 8,077.5	$ 7,895.9	$ 6,668.2
(Loss) income from continuing operations before income taxes, extraordinary items, noncontrolling interest									208.6	(1,376.0)	(1,138.7)
Inventories	548.4				444.4				548.4	444.4
Property, plant, equipment and mine development, net	11,801.7				11,251.6				11,801.7	11,251.6	7,263.3
Accounts payable and accrued expenses	1,606.9				1,712.3				1,606.9	1,712.3
Recognized gain on coal reserves exchange										37.7	23.7
Cash proceeds from sale of non-strategic coal reserves										24.9
Notes receivables from sale of nonstrategic coal reserves										17.4
Gain recognized										31.7
Prarie State Energy Campus [Member]
Resource Management and Other Commercial Events (Textuals) [Abstract]
Undivided Interest Percent Of New Electricity Generation Project	5.06%								5.06%
Revenues									4.0
(Loss) income from continuing operations before income taxes, extraordinary items, noncontrolling interest									6.3
Capacity of new electricity generation project, Megawatts									1,600
Commencementof800megawattgenerationunit			800
Commencementof800Megawattgenerationunit2	800
Inventories	3.6								3.6
Property, plant, equipment and mine development, net	243.8								243.8
Accounts payable and accrued expenses	1.4								1.4
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	246.0								246.0
Project development fee received									$ 14.6