Fair Value Measurement (Details) (Assets and liabilities measured on a recurring basis, Commodity Contract, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivatives asset (liability), net		$ (22.7)
Commodity derivatives asset (liability), net	23.2
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivatives asset (liability), net		0
Commodity derivatives asset (liability), net	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivatives asset (liability), net		(22.7)
Commodity derivatives asset (liability), net	23.2
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivatives asset (liability), net		0
Commodity derivatives asset (liability), net	$ 0