Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating segment results
Revenues	$ 1,748.0	$ 2,020.7
Adjusted EBITDA	280.1	511.5
Reconciliation Of Adjusted EBITDA To Consolidated (Loss) Income From Continuing Operations, Net of Income Taxes [Abstract]
Adjusted EBITDA	280.1	511.5
Depreciation, depletion and amortization	(170.7)	(142.9)
Amortization of basis difference related to equity affiliates	1.6	2.3
Asset retirement obligation expenses	19.0	15.0
Interest expense	101.3	102.0
Interest income	(5.9)	(8.1)
Income tax provision	3.7	74.4
(Loss) income from continuing operations, net of income taxes	(10.3)	183.0
Western U.S. Mining [Member]
Operating segment results
Revenues	640.1	761.0
Adjusted EBITDA	160.5	207.0
Reconciliation Of Adjusted EBITDA To Consolidated (Loss) Income From Continuing Operations, Net of Income Taxes [Abstract]
Adjusted EBITDA	160.5	207.0
Midwestern U.S. Mining [Member]
Operating segment results
Revenues	336.7	347.7
Adjusted EBITDA	112.3	109.2
Reconciliation Of Adjusted EBITDA To Consolidated (Loss) Income From Continuing Operations, Net of Income Taxes [Abstract]
Adjusted EBITDA	112.3	109.2
Australian Mining [Member]
Operating segment results
Revenues	738.0	854.1
Adjusted EBITDA	100.4	295.6
Reconciliation Of Adjusted EBITDA To Consolidated (Loss) Income From Continuing Operations, Net of Income Taxes [Abstract]
Adjusted EBITDA	100.4	295.6
Trading and Brokerage [Member]
Operating segment results
Revenues	25.9	52.5
Adjusted EBITDA	16.0	28.1
Reconciliation Of Adjusted EBITDA To Consolidated (Loss) Income From Continuing Operations, Net of Income Taxes [Abstract]
Adjusted EBITDA	16.0	28.1
Corporate and Other [Member]
Operating segment results
Revenues	7.3	5.4
Adjusted EBITDA	(109.1)	(128.4)
Reconciliation Of Adjusted EBITDA To Consolidated (Loss) Income From Continuing Operations, Net of Income Taxes [Abstract]
Adjusted EBITDA	$ (109.1)	$ (128.4)