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Fair Value of Financial Instruments (Details) (USD $)
9 Months Ended
Sep. 30, 2012
Debt_Instruments
Dec. 31, 2011
Liabilities [Abstract]    
Long-term debt, excluding current maturities $ 4,820,000,000 $ 4,820,000,000
Number of debt instruments with related interest rate swaps (debt instruments) 2  
Notional amounts of interest rate swaps 1,000,000,000  
Floating rate basis LIBOR-based  
Weighted average floating rate (percentage) 3.40%  
Aggregate fixed rate debt after effect of interest rate swaps 3,800,000,000  
Aggregate floating rate debt after effect of interest rate swaps 1,000,000,000  
Fair Value
   
Liabilities [Abstract]    
Long-term Debt, Fair Value 6,500,000,000 6,200,000,000
Carrying Value
   
Liabilities [Abstract]    
Long-term debt, excluding current maturities 4,800,000,000 4,800,000,000
Fair Value, Inputs, Level 1
   
Liabilities [Abstract]    
Long-term Debt, Fair Value 3,800,000,000 3,600,000,000
Fair Value, Inputs, Level 2
   
Liabilities [Abstract]    
Long-term Debt, Fair Value $ 2,700,000,000 $ 2,600,000,000