Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash Flows From Operating Activities
Net (loss) income	$ (19.4)	$ 178.3
Loss from discontinued operations, net of income taxes	9.1	4.7
(Loss) income from continuing operations, net of income taxes	(10.3)	183.0
Adjustments to reconcile (loss) income from continuing operations, net of income taxes to net cash provided by operating activities:
Depreciation, depletion and amortization	170.7	142.9
Noncash interest expense	5.6	4.5
Deferred income taxes	(47.8)	0.7
Share-based compensation	11.8	13.0
Net gain on disposal or exchange of assets	(2.6)	(4.0)
Loss from equity affiliates	17.6	22.7
Changes in current assets and liabilities:
Accounts receivable	77.5	261.6
Change in receivable from accounts receivable securitization program	0	(75.0)
Inventories	(23.3)	(78.8)
Net assets from coal trading activities	25.3	31.6
Other current assets	7.1	6.2
Accounts payable and accrued expenses	41.7	(108.7)
Asset retirement obligations	12.7	13.3
Accrued postretirement benefit costs	3.8	7.3
Pension costs	12.9	12.4
Contributions to pension plans	(0.4)	(0.4)
Other, net	(14.8)	(1.9)
Net cash provided by continuing operations	287.5	430.4
Net cash used in discontinued operations	(15.8)	(34.9)
Net cash provided by operating activities	271.7	395.5
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(74.0)	(234.5)
Changes in accrued expenses related to capital expenditures	(66.4)	0
Investment in Prairie State Energy Campus	0	(3.3)
Proceeds from disposal of assets, net of note receivable	53.0	4.6
Purchases of debt securities	(4.6)	0
Proceeds from sales and maturities of debt securities	12.9	3.1
Contributions to joint ventures	(154.1)	(208.9)
Distributions from joint ventures	174.3	221.4
Advances to related parties	(23.1)	(322.0)
Repayment of loans from related parties	14.8	338.7
Other, net	(1.0)	(0.5)
Net cash used in continuing operations	(68.2)	(201.4)
Net cash used in discontinued operations	(0.1)	(1.7)
Net cash used in investing activities	(68.3)	(203.1)
Cash Flows From Financing Activities
Repayments of long-term debt	(108.5)	(13.4)
Dividends paid	(22.9)	(23.2)
Repurchase of employee common stock relinquished for tax withholding	(2.6)	(7.9)
Excess tax benefits related to share-based compensation	0	3.5
Other, net	1.3	1.9
Net cash used in financing activities	(132.7)	(39.1)
Net change in cash and cash equivalents	70.7	153.3
Cash and cash equivalents at beginning of period	558.8	799.1
Cash and cash equivalents at end of period	629.5	952.4
Retained Earnings [Member]
Cash Flows From Operating Activities
Net (loss) income	$ (23.4)