Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 M	Dec. 31, 2011	Dec. 31, 2010
Cash and Cash Equivalents
Maximum original maturity period of short-term deposits and highly liquid investments to be considered as cash equivalents (in months)	3
Owned and leased mineral rights
Depletion expense included in depreciation and depletion	$ 99.8	$ 59.3	$ 2.5
Mineral rights | Low end of range
Owned and leased mineral rights
Original lease term (in years)	10
Mineral rights | High end of range
Owned and leased mineral rights
Original lease term (in years)	50