Supplemental Guarantor/Non-Guarantor Financial Information (Details 2) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Cash Flows From Operating Activities
Net cash provided by (used In) continuing operations	$ 1,373.7	$ 1,216.7
Net cash (used in) provided by discontinued operations	(82.2)	(26.2)
Net cash provided by (used in) operating activities	1,291.5	1,190.5
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(732.1)	(553.4)
Federal coal lease expenditures	(247.9)	(42.4)
Investment in Prairie State Energy Campus	(9.4)	(29.8)
Proceeds from disposal of assets	93.5	12.1
Investments in equity affiliates and joint ventures	0	(39.8)
Proceeds from sales and maturities of debt and equity securities	39.0	53.3
Purchases of debt and equity securities	(23.8)	(44.1)
Purchases of short-term investments	0	(100.0)
Proceeds from sale of short-term investments		100.0
Investment in shares of Macarthur Coal Limited	0	(45.5)
Contributions to joint ventures	(531.2)	0
Distributions from joint ventures	527.7	0
Repayment of loans from related parties	720.2	0
Advances to related parties	(743.4)	0
Other, net	(3.2)	(4.4)
Net cash used in continuing operations	(910.6)	(694.0)
Net cash used in discontinued operations	(11.2)	(62.8)
Net cash used in investing activities	(921.8)	(756.8)
Cash Flows From Financing Activities
Payments of long-term debt	(305.7)	(248.4)
Common stock repurchase	(99.9)	0
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests	(49.8)	0
Dividends paid	(69.1)	(69.1)
Repurchase of employee common stock relinquished for tax withholding	(8.3)	(16.1)
Excess tax benefits related to share-based compensation	3.6	6.1
Other, net	(8.4)	(0.2)
Transactions with affiliates, net	0	0
Net cash (used in) provided by financing activities	(520.8)	(327.3)
Net change in cash and cash equivalents	(151.1)	106.4
Cash and cash equivalents at beginning of period	799.1	1,295.2
Cash and cash equivalents at end of period	648.0	1,401.6
Parent Company [Member]
Cash Flows From Operating Activities
Net cash provided by (used In) continuing operations	252.2	144.7
Net cash (used in) provided by discontinued operations	(1.6)	(1.6)
Net cash provided by (used in) operating activities	250.6	143.1
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	0	0
Federal coal lease expenditures	0	0
Investment in Prairie State Energy Campus	0	0
Proceeds from disposal of assets	0	0
Investments in equity affiliates and joint ventures		0
Proceeds from sales and maturities of debt and equity securities	0	0
Purchases of debt and equity securities	0	0
Purchases of short-term investments		(75.0)
Proceeds from sale of short-term investments		75.0
Investment in shares of Macarthur Coal Limited		0
Contributions to joint ventures	0
Distributions from joint ventures	0
Repayment of loans from related parties	0
Advances to related parties	0
Other, net	0	0
Net cash used in continuing operations	0	0
Net cash used in discontinued operations	0	0
Net cash used in investing activities	0	0
Cash Flows From Financing Activities
Payments of long-term debt	(285.3)	(236.9)
Common stock repurchase	(99.9)
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests	0
Dividends paid	(69.1)	(69.1)
Repurchase of employee common stock relinquished for tax withholding	(8.3)	(16.1)
Excess tax benefits related to share-based compensation	3.6	6.1
Other, net	(9.4)	(6.8)
Transactions with affiliates, net	153.7	220.2
Net cash (used in) provided by financing activities	(295.9)	(89.0)
Net change in cash and cash equivalents	(45.3)	54.1
Cash and cash equivalents at beginning of period	283.2	903.8
Cash and cash equivalents at end of period	237.9	957.9
Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash provided by (used In) continuing operations	1,225.0	718.3
Net cash (used in) provided by discontinued operations	(8.0)	1.2
Net cash provided by (used in) operating activities	1,217.0	719.5
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(188.3)	(211.1)
Federal coal lease expenditures	(247.9)	(42.4)
Investment in Prairie State Energy Campus	(9.4)	(29.8)
Proceeds from disposal of assets	59.3	12.1
Investments in equity affiliates and joint ventures		(2.1)
Proceeds from sales and maturities of debt and equity securities	0	0
Purchases of debt and equity securities	0	0
Purchases of short-term investments		0
Proceeds from sale of short-term investments		0
Investment in shares of Macarthur Coal Limited		0
Contributions to joint ventures	0
Distributions from joint ventures	0
Repayment of loans from related parties	0
Advances to related parties	0
Other, net	(3.0)	(4.4)
Net cash used in continuing operations	(389.3)	(277.7)
Net cash used in discontinued operations	(1.9)	(7.5)
Net cash used in investing activities	(391.2)	(285.2)
Cash Flows From Financing Activities
Payments of long-term debt	(0.3)	0
Common stock repurchase	0
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests	0
Dividends paid	0	0
Repurchase of employee common stock relinquished for tax withholding	0	0
Excess tax benefits related to share-based compensation	0	0
Other, net	0	0
Transactions with affiliates, net	(826.1)	(438.5)
Net cash (used in) provided by financing activities	(826.4)	(438.5)
Net change in cash and cash equivalents	(0.6)	(4.2)
Cash and cash equivalents at beginning of period	0.8	5.2
Cash and cash equivalents at end of period	0.2	1.0
Non-Guarantor Subsidiaries [Member]
Cash Flows From Operating Activities
Net cash provided by (used In) continuing operations	(103.5)	353.7
Net cash (used in) provided by discontinued operations	(72.6)	(25.8)
Net cash provided by (used in) operating activities	(176.1)	327.9
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(543.8)	(342.3)
Federal coal lease expenditures	0	0
Investment in Prairie State Energy Campus	0	0
Proceeds from disposal of assets	34.2	0
Investments in equity affiliates and joint ventures		(37.7)
Proceeds from sales and maturities of debt and equity securities	39.0	53.3
Purchases of debt and equity securities	(23.8)	(44.1)
Purchases of short-term investments		(25.0)
Proceeds from sale of short-term investments		25.0
Investment in shares of Macarthur Coal Limited		(45.5)
Contributions to joint ventures	(531.2)
Distributions from joint ventures	527.7
Repayment of loans from related parties	720.2
Advances to related parties	(743.4)
Other, net	(0.2)	0
Net cash used in continuing operations	(521.3)	(416.3)
Net cash used in discontinued operations	(9.3)	(55.3)
Net cash used in investing activities	(530.6)	(471.6)
Cash Flows From Financing Activities
Payments of long-term debt	(20.1)	(11.5)
Common stock repurchase	0
Acquisition of MCG Coal Holdings Pty Ltd noncontrolling interests	(49.8)
Dividends paid	0	0
Repurchase of employee common stock relinquished for tax withholding	0	0
Excess tax benefits related to share-based compensation	0	0
Other, net	1.0	6.6
Transactions with affiliates, net	672.4	218.3
Net cash (used in) provided by financing activities	601.5	200.2
Net change in cash and cash equivalents	(105.2)	56.5
Cash and cash equivalents at beginning of period	515.1	386.2
Cash and cash equivalents at end of period	$ 409.9	$ 442.7