Income and Mining Tax Benefit (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Tax Disclosure [Abstract]
Total income and mining taxes expenses (benefit)	$ 2.0	$ (10.8)	$ (1.2)	$ (22.9)
Out-of-period adjustment			$ (1.8)