Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 Parent Company [Member]	Sep. 30, 2010 Parent Company [Member]	Sep. 30, 2011 Guarantor Subsidiaries [Member]	Sep. 30, 2010 Guarantor Subsidiaries [Member]	Sep. 30, 2011 Non-Guarantor Subsidiaries [Member]	Sep. 30, 2010 Non-Guarantor Subsidiaries [Member]	Sep. 30, 2011 April 30, 2016 [Member]	Apr. 15, 2011 April 30, 2016 [Member]	Sep. 30, 2011 November 30, 2016 [Member]	Sep. 30, 2011 September 30, 2020 [Member]	Sep. 30, 2011 November 30, 2026 [Member]	Sep. 30, 2011 March 31, 2013 [Member]
Cash Flows From Operating Activities
Net cash provided by continuing operations		$ 1,194.1	$ 903.5	$ 144.7	$ (323.0)	$ 721.5	$ 1,002.7	$ 327.9	$ 223.8
Net cash used in discontinued operations		(3.6)	(11.3)	(1.6)	(9.5)	(2.0)	(1.8)	0	0
Net cash provided by operating activities		1,190.5	892.2	143.1	(332.5)	719.5	1,000.9	327.9	223.8
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development		(616.2)	(291.3)	0	0	(218.6)	(238.1)	(397.6)	(53.2)
Federal coal lease expenditures		(42.4)	0	0		(42.4)		0
Investment in Prairie State Energy Campus		(29.8)	(52.5)	0	0	(29.8)	(52.5)	0	0
Proceeds from disposal of assets		12.1	9.7	0	0	12.1	8.5	0	1.2
Investments in equity affiliates and joint ventures		(39.8)	(18.8)	0	0	(2.1)	(15.0)	(37.7)	(3.8)
Proceeds from sales of debt and equity securities		53.3	10.6	0	0	0	0	53.3	10.6
Purchases of debt and equity securities		(44.1)	(73.6)		0		0	(44.1)	(73.6)
Purchases of short-term investments		(100.0)	0	(75.0)		0		(25.0)
Maturity of short-term investments		100.0	0	75.0		0		25.0
Investment in shares of Macarthur Coal Limited	(45.5)	(45.5)	0	0		0		(45.5)
Other, net		(4.4)	(7.4)	0	0	(4.4)	(7.2)	0	(0.2)
Net cash used in investing activities		(756.8)	(423.3)	0	0	(285.2)	(304.3)	(471.6)	(119.0)
Cash Flows From Financing Activities
Proceeds from long-term debt		1.4	1,150.0	0	1,150.0	0	0	1.4	0
Payments of long-term debt		(248.4)	(1,148.5)	(236.9)	(1,140.3)	0	0	(11.5)	(8.2)
Dividends paid		(69.1)	(56.5)	(69.1)	(56.5)	0	0	0	0
Repurchase of employee common stock relinquished for tax withholding		(16.1)	(8.4)	(16.1)	(8.4)	0	0	0	0
Payment of debt issuance costs		(4.0)	(32.2)	(4.0)	(32.2)	0	0	0	0
Excess tax benefits related to share-based compensation		6.1	0	6.1		0		0
Proceeds from stock options exercised		4.5	5.9	4.5	5.9	0	0	0	0
Other, net		(1.7)	(0.5)	6.3	5.8	0	0	(8.0)	(6.3)
Transactions with affiliates, net		0	0	220.2	521.1	(438.5)	(696.7)	218.3	175.6
Net cash used in financing activities		(327.3)	(90.2)	(89.0)	445.4	(438.5)	(696.7)	200.2	161.1
Net change in cash and cash equivalents		106.4	378.7	54.1	112.9	(4.2)	(0.1)	56.5	265.9
Cash and cash equivalents at beginning of period		1,295.2	988.8	903.8	368.4	5.2	0.2	386.2	620.2
Cash and cash equivalents at end of period	$ 1,401.6	$ 1,401.6	$ 1,367.5	$ 957.9	$ 481.3	$ 1.0	$ 0.1	$ 442.7	$ 886.1
Supplemental Guarantor/Non-Guarantor Financial Information (Textuals)
Interest rates on Senior Notes										5.875%	5.875%	7.375%	6.50%	7.875%	6.875%