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Fair Value Measurement (Details 2) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Changes in Level 3 fair value measurements      
Fair value liability, beginning of period $ 0 $ 101.8 $ 26.0
Transfers out of Level 3 0 [1] (101.8) [1] 0 [1]
Realized and unrealized (gain) loss included in earnings 0 0 37.4
Settlements 0 0 38.4
Fair value liability, end of period 0 0 101.8
Fair Value, Inputs, Level 3
     
Changes in Level 3 fair value measurements      
Total unrealized (gain) loss included in income related to financial assets and liabilities still on the Balance Sheets $ 0 $ 0 $ 75.8
[1] During the first quarter of 2011, the inputs used to value oil collars, natural gas collars and natural gas basis swaps were directly or indirectly observable, and these instruments were transferred to level 2.