Fair Value Measurements (Details) (Commodity contract, Assets and liabilities measured on a recurring basis, USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Estimated Fair Value
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives liability, net	$ (102.4)	$ (113.6)
Level 1
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives liability, net	0	0
Level 2
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives liability, net	(102.4)	(11.8)
Level 3
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives liability, net		$ (101.8)