Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2010
Asset Retirement Obligation Disclosure [Abstract]
Credit adjusted, risk-free interest rates	6.28%	5.76%		6.37%
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities	$ 571.6	$ 791.6
Amount of reclamation self-bonding in certain states in which the company qualifies	1,275.8	929.6
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	615.2	484.9	687.5
Liabilities incurred or acquired	13.6	43.0
Liabilities settled or disposed	(12.8)	(8.7)
Accretion expense	34.3	28.3
Revision of estimates	37.2	67.7
Balance at end of year — active locations	629.1	567.9
Balance at end of year — closed or inactive locations	$ 58.4	$ 47.3