Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Cash Flows From Operating Activities
Net income	$ 178.3	$ 178.7
Loss from discontinued operations, net of income taxes	4.0	15.9
Income from continuing operations, net of income taxes	182.3	194.6
Adjustments to reconcile income from continuing operations, net of income taxes to net cash provided by operating activities:
Depreciation, depletion and amortization	144.9	107.7
Deferred income taxes	0.3	23.9
Share-based compensation	13.0	10.9
Net gain on disposal or exchange of assets	(4.0)	(4.0)
Loss (income) from equity affiliates	22.7	3.0
Changes in current assets and liabilities:
Accounts receivable	261.6	81.8
Change in receivable from accounts receivable securitization program	(75.0)	0
Inventories	(78.8)	(18.2)
Net assets from coal trading activities	31.6	(72.8)
Other current assets	6.2	(9.2)
Accounts payable and accrued expenses	(108.7)	(102.7)
Asset retirement obligations	13.4	7.3
Workers' compensation obligations	1.8	7.1
Pension costs	12.4	4.0
Accrued postretirement benefit costs	7.3	6.3
Contributions to pension plans	(0.4)	(0.4)
Other, net	0.8	(4.1)
Net cash provided by continuing operations	431.4	235.2
Net cash used in discontinued operations	(35.9)	(14.6)
Net cash provided by operating activities	395.5	220.6
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(235.3)	(93.6)
Investment in Prairie State Energy Campus	(3.3)	(8.9)
Proceeds from disposal of assets	4.6	5.5
Investments in equity affiliates and joint ventures	0	(1.1)
Proceeds from sales and maturities of debt and equity securities	3.1	15.5
Purchases of debt and equity securities	0	(14.6)
Purchases of short-term investments	0	(100.0)
Contributions to joint ventures	(208.9)	0
Distributions from joint ventures	221.4	0
Repayment of loans from related parties	338.7	0
Advances to related parties	(322.0)	0
Other, net	(0.5)	(0.4)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(202.2)	(197.6)
Cash Provided by (Used in) Investing Activities, Discontinued Operations	(0.9)	(13.6)
Net cash used in investing activities	(203.1)	(211.2)
Cash Flows From Financing Activities
Payments of long-term debt	(13.4)	(10.0)
Dividends paid	(23.2)	(23.0)
Repurchase of employee common stock relinquished for tax withholding	(7.9)	(15.1)
Excess tax benefits related to share-based compensation	3.5	4.9
Proceeds from Stock options exercised	1.4	4.0
Other, net	0.5	7.8
Net cash used in financing activities	(39.1)	(31.4)
Net change in cash and cash equivalents	153.3	(22.0)
Cash and cash equivalents at beginning of period	799.1	1,295.2
Cash and cash equivalents at end of period	952.4	1,273.2
Retained Earnings [Member]
Cash Flows From Operating Activities
Net income	172.7
Treasury Stock [Member]
Cash Flows From Operating Activities
Net income	$ 0