Acquisition of Macarthur Coal Limited (Pro Forma Information) (Details 3) (Macarthur [Member], USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Macarthur [Member]
Business Acquisition [Line Items]
Revenue	$ 8,618.0	$ 7,387.7
Income from continuing operations, net of income taxes	$ 1,305.7	$ 1,002.4
Basic earnings per share	$ 3.55	$ 2.65
Diluted earnings per share	$ 3.55	$ 2.65