Consolidated Statements of Cash Flows (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash Flows From Operating Activities
Net (loss) income	$ (575.1)	$ 946.3	$ 802.2
Loss (income) from discontinued operations, net of income taxes	104.2	66.5	22.8
(Loss) Income from continuing operations, net of income taxes	(470.9)	1,012.8	825.0
Adjustments to reconcile income from continuing operations, net of income taxes to net cash provided by operating activities:
Depreciation, depletion and amortization	663.4	474.3	429.5
Noncash interest expense	20.7	15.5	12.8
Deferred income taxes	(2.6)	180.6	69.0
Share-based compensation	45.4	43.9	41.1
Asset impairment and mine closure costs	929.0	0	0
Monetization of foreign currency hedge positions	151.8	0	0
Net gain on disposal or exchange of assets	(17.1)	(76.9)	(29.9)
Loss from equity affiliates	61.2	19.2	1.7
Changes in current assets and liabilities:
Accounts receivable	285.3	(221.0)	(149.4)
Change in receivable from accounts receivable securitization program	(125.0)	0	(104.6)
Inventories	(112.9)	(50.4)	(8.5)
Net assets from coal trading activities	125.2	172.4	(109.6)
Other current assets	10.3	(27.4)	(28.5)
Accounts payable and accrued expenses	(117.1)	83.3	222.9
Asset retirement obligations	46.4	30.4	29.9
Workers' compensation obligations	9.4	10.4	(8.9)
Accrued postretirement benefit costs	38.9	35.4	23.1
Pension costs	34.1	31.1	23.3
Contributions to pension plans	(1.7)	(46.7)	(112.6)
Other, net	26.0	(34.8)	(21.8)
Net cash provided by continuing operations	1,599.8	1,652.1	1,104.5
Net cash used in discontinued operations	(84.7)	(18.9)	(17.4)
Net cash provided by operating activities	1,515.1	1,633.2	1,087.1
Cash Flows From Investing Activities
Additions to property, plant, equipment and mine development	(986.0)	(847.4)	(546.0)
Changes in accrued expenses related to capital expenditures	104.7	51.2	0
Federal Coal Lease Expenditures	(276.5)	(42.4)	0
Investment in Prairie State Energy Campus	(10.7)	(36.2)	(76.0)
Proceeds from disposal of assets, net of notes receivable	147.9	40.1	19.1
Investments in equity affiliates and joint ventures	0	(39.7)	(18.8)
Purchases of debt and equity securities	(46.7)	(147.7)	(74.6)
Proceeds from sales of debt and equity securities	46.4	104.6	12.4
Purchases of short-term investments	(4.8)	(100.0)	0
Maturity of short-term investments	0	100.0	0
Acquisition of Macarthur Coal Limited, net of cash acquired	0	(2,756.7)	0
Contributions to joint ventures	(824.0)	(145.4)	0
Distributions from joint ventures	823.0	128.6	0
Advances to related parties	(148.0)	(371.3)	0
Repayment of loan from related parties	110.8	331.7	0
Other, net	(6.2)	(6.6)	(8.8)
Net cash used in continuing operations	(1,070.1)	(3,737.2)	(692.7)
Net cash (used in) provided by discontinued operations	(22.0)	(70.6)	(10.9)
Net cash used in investing activities	(1,092.1)	(3,807.8)	(703.6)
Cash Flows From Financing Activities
Repayments of long-term debt	(415.8)	(263.9)	(1,167.3)
Proceeds from long-term debt	0.8	4,101.4	1,150.0
Payment of debt issuance costs	(6.9)	(61.5)	(32.2)
Dividends paid	(91.9)	(92.1)	(79.4)
Common stock repurchase	(99.9)	0	0
Repurchase of employee common stock relinquished for tax withholding	(8.4)	(18.7)	(13.5)
Excess tax benefit related to share-based compensation	8.3	8.1	51.0
Acqiusition of MCG Coal Holdings Pty Ltd noncontrolling interests	49.8	0	0
Acquisition of PEAMCoal Pty Ltd noncontrolling interests	0	(1,994.8)	0
Other, net	0.3	0	14.3
Net cash (used in) provided by financing activities	(663.3)	1,678.5	(77.1)
Net change in cash and cash equivalents	(240.3)	(496.1)	306.4
Cash and cash equivalents at beginning of year	799.1	1,295.2	988.8
Cash and cash equivalents at end of year	$ 558.8	$ 799.1	$ 1,295.2