Fair Value Measurement (Details) (Assets (Liabilities) Measured at Fair Value on a Recurring Basis, Commodity contract, USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	$ 19.0
Commodity derivative liability, net		(22.7)
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	0
Commodity derivative liability, net		0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	19.0
Commodity derivative liability, net		(22.7)
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	0
Commodity derivative liability, net		$ 0