Fair Value Measurement (Details) (Assets and liabilities measured on a recurring basis, Commodity contract, USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative liability, net	$ 63.5	$ 22.7
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative liability, net	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative liability, net	63.5	22.7
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative liability, net	$ 0	$ 0