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Fair Value Measurement (Details 2) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Changes in Level 3 fair value measurements      
Fair value (asset), beginning of period     $ (172.5)
Fair value liability, beginning of period 101.8 26.0  
Transfers out of Level 3 (101.8) [1] 0 [1] (3.4) [1]
Realized and unrealized (gain) loss included in earnings 0 37.4 (71.0)
Unrealized (gain) loss included in accumulated other comprehensive loss 0 0 201.9
Settlements 0 38.4 71.0
Fair value liability, end of period 0 101.8 26.0
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 $ 75.8 $ (0.4)
[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.