Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Cash Flows From Operating Activities
Net income	$ 427.7	$ 752.4
Loss from discontinued operations, net of income taxes	92.7	38.9
Income from continuing operations, net of income taxes	520.4	791.3
Adjustments to reconcile income from continuing operations, net of income taxes to net cash provided by operating activities:
Depreciation, depletion and amortization	470.7	317.6
Deferred income taxes	(162.6)	48.2
Share-based compensation	34.9	32.4
Net gain on disposal or exchange of assets	(7.6)	(31.4)
Loss from equity affiliates	50.5	9.0
Changes in current assets and liabilities:
Accounts receivable	259.0	(11.8)
Change in receivable from accounts receivable securitization program	(50.0)	0
Inventories	(117.4)	(12.3)
Net assets from coal trading activities	145.6	71.7
Other current assets	42.7	(42.3)
Accounts payable and accrued expenses	84.6	(15.6)
Asset retirement obligations	55.6	27.2
Accrued postretirement benefit costs	27.7	26.6
Pension costs	26.0	25.5
Contributions to pension plans	(1.3)	(1.3)
Other, net	(5.1)	(18.1)
Net cash provided by continuing operations	1,373.7	1,216.7
Net cash used in discontinued operations	(82.2)	(26.2)
Net cash provided by 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	0	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
Net cash used in 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
Retained Earnings [Member]
Cash Flows From Operating Activities
Net income	420.3
Cash Flows From Financing Activities
Dividends paid	(69.1)
Treasury Stock [Member]
Cash Flows From Financing Activities
Common stock repurchase	$ (99.9)