Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating segment results
Revenues	$ 2,038.6	$ 1,743.1
Adjusted EBITDA	512.6	435.3
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	512.6	435.3
Depreciation, depletion and amortization	(144.9)	(107.7)
Loss from equity affiliates: amortization of basis difference	2.3	0
Asset retirement obligation expense	15.2	13.3
Interest expense	102.0	51.0
Interest income	(8.1)	(4.1)
Income tax provision	74.0	72.8
Income from continuing operations, net of income taxes	182.3	194.6
Western U.S. Mining [Member]
Operating segment results
Revenues	761.0	703.7
Adjusted EBITDA	207.0	179.4
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	207.0	179.4
Midwestern U.S. Mining [Member]
Operating segment results
Revenues	365.6	367.0
Adjusted EBITDA	110.3	109.9
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	110.3	109.9
Australian Mining [Member]
Operating segment results
Revenues	854.1	578.8
Adjusted EBITDA	295.6	209.6
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	295.6	209.6
Trading and Brokerage [Member]
Operating segment results
Revenues	52.5	83.9
Adjusted EBITDA	28.1	26.8
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	28.1	26.8
Corporate and Other [Member]
Operating segment results
Revenues	5.4	9.7
Adjusted EBITDA	(128.4)	(90.4)
Reconciliation Of Adjusted Ebitda To Consolidated Income From Continuing Operations [Abstract]
Adjusted EBITDA	$ (128.4)	$ (90.4)