Asset Impairment and Mine Closure Costs (Details) (USD $)	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets to be Disposed of			$ 26,900,000
Decrease in Seaborne Coal Prices	30.00%		30.00%
Impairment of long-lived assets	921,300,000	7,700,000	929,000,000	0	0
Asset Impairment Charges				0	0
Asset Retirement Obligation, Revision of Estimate			7,100,000
Severance Costs			6,700,000
Other			4,300,000
Long-lived assets held and used			806,700,000
Marketable securities			35,500,000	0	0
Cost method investments			39,400,000
Other			2,400,000
Australian Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets to be Disposed of			0
Impairment of long-lived assets			806,700,000
Asset Retirement Obligation, Revision of Estimate			0
Severance Costs			0
Other			0
Long-lived assets held and used			806,700,000
Marketable securities			0
Cost method investments			0
Other			0
Western U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets to be Disposed of			0
Impairment of long-lived assets			2,400,000
Asset Retirement Obligation, Revision of Estimate			0
Severance Costs			0
Other			0
Long-lived assets held and used			0
Marketable securities			0
Cost method investments			0
Other			2,400,000
Midwestern U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets to be Disposed of			26,900,000
Impairment of long-lived assets			45,000,000
Asset Impairment Charges			(45,000,000)
Asset Retirement Obligation, Revision of Estimate			7,100,000
Severance Costs			6,700,000
Other			4,300,000
Long-lived assets held and used			0
Marketable securities			0
Cost method investments			0
Other			0
Corporate and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets to be Disposed of			0
Impairment of long-lived assets			74,900,000
Asset Retirement Obligation, Revision of Estimate			0
Severance Costs			0
Other			0
Long-lived assets held and used			0
Marketable securities			35,500,000
Cost method investments			39,400,000
Other			$ 0