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Fair Value of Financial Instruments (Details) (USD $)
6 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2011
Debt_Instruments
Assets [Abstract]      
Marketable securities $ 50,000,000    
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) 3.50%    
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 6,300,000,000 6,200,000,000  
Carrying Value
     
Liabilities [Abstract]      
Long-term Debt, Excluding Current Maturities $ 4,800,000,000 $ 4,800,000,000