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General Accounting Matters (Details) (USD $)
In Millions
6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Dec. 31, 2010
Activity of allowance for doubtful accounts [Roll Forward]      
Balance at beginning of period $ 18.4 $ 16.8  
Charged to costs and expenses 0.6 0.7  
Deductions (5.2) [1] 0 [1]  
Balance at end of period 13.8 17.5  
Liquids exchange contracts [Abstract]      
Net payable liquids exchange balance 212.4   144.1
Restricted cash [Abstract]      
Restricted cash 116.2   98.7
Fair Value [Member]
     
Financial liabilities:      
Fixed-rate debt (principal amount) $ 14,090   $ 12,910
[1] Primarily due to our reassessment of the allowance for doubtful accounts as a result of improved credit ratings of a significant customer, which reduced our exposure to potential uncollectibility.