Fair Value Measurement (Details) (Assets (Liabilities) Measured at Fair Value on a Recurring Basis, Commodity Contract, USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	$ 53.2	$ 23.2
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	53.2	23.2
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis
Commodity derivative asset, net	$ 0	$ 0