Summary of Significant Accounting Policies Discussion (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Other than Temporary Impairment	$ 0	$ 0	$ 0
Equity method investments
Book Value	298.6	526.6
Income (loss) from equity affiliates	(61.2)	(19.2)	(1.7)
Interest In Middlemount Coal Pty Limited [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	143.1
Equity method investments
Book Value	295.9	449.7
Income (loss) from equity affiliates	52.1	7.3	0
Interest In Carbones Del Guasare [Member]
Equity method investments
Book Value	0
Other Equity Method Investments [Member]
Equity method investments
Book Value	2.7	76.9
Income (loss) from equity affiliates	$ 9.1	$ 11.9	$ (1.7)