Acquisition of Macarthur Coal Limited Details 1 (Details) (Macarthur [Member], USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011
Macarthur [Member]
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Revenue	$ 2,194.1	$ 6,154.7
Business Acquisition, Pro Forma Net Income (Loss)	$ 256.0	$ 727.6
Business Acquisition, Pro Forma Earnings Per Share, Basic	$ 0.88	$ 2.48
Business Acquisition, Pro Forma Earnings Per Share, Diluted	$ 0.87	$ 2.48