Segment Information (Details) (USD $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Operating segment results
Revenues	$ 2,008.0	$ 1,661.4	$ 3,752.9	$ 3,177.0
Adjusted EBITDA	579.5	440.4	995.7	797.6
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	579.5	440.4	995.7	797.6
Depreciation, depletion and amortization	105.3	105.1	214.1	210.6
Asset retirement obligation expense	15.8	10.9	28.9	20.4
Interest expense	49.1	57.9	100.1	107.9
Interest income	(3.5)	(1.6)	(7.6)	(2.6)
Income tax provision	119.8	53.4	187.6	109.5
Income from continuing operations, net of income taxes	293.0	214.7	472.6	351.8
Western U.S. Mining [Member]
Operating segment results
Revenues	662.3	652.1	1,366.0	1,314.2
Adjusted EBITDA	134.1	207.3	313.5	415.2
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	134.1	207.3	313.5	415.2
Midwestern U.S. Mining [Member]
Operating segment results
Revenues	327.9	323.3	694.9	632.7
Adjusted EBITDA	82.0	71.4	191.9	145.5
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	82.0	71.4	191.9	145.5
Australian Mining [Member]
Operating segment results
Revenues	885.3	597.4	1,465.9	1,043.9
Adjusted EBITDA	376.9	223.6	567.4	346.9
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	376.9	223.6	567.4	346.9
Trading and Brokerage [Member]
Operating segment results
Revenues	114.1	81.8	198.0	171.9
Adjusted EBITDA	50.4	14.3	77.2	46.7
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	50.4	14.3	77.2	46.7
Corporate and Other [Member]
Operating segment results
Revenues	18.4	6.8	28.1	14.3
Adjusted EBITDA	(63.9)	(76.2)	(154.3)	(156.7)
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	$ (63.9)	$ (76.2)	$ (154.3)	$ (156.7)

Reconciliation of Adjusted EBITDA to consolidated income from continuing operations.

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Segment Reporting Information Adjusted Earning Before Interest Taxes Depreciation And Amortization.

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Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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