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Fair Value of Financial Instruments (Details) (USD $)
3 Months Ended
Mar. 31, 2012
Jun. 30, 2011
Mar. 31, 2012
Carrying Value [Member]
Dec. 31, 2011
Carrying Value [Member]
Mar. 31, 2012
Fair Value [Member]
Dec. 31, 2011
Fair Value [Member]
Assets [Abstract]            
Marketable securities $ 100,000,000          
Liabilities [Abstract]            
Long-term debt     4,800,000,000 4,800,000,000 6,000,000,000 6,200,000,000
Number of debt instruments with related interest rate swaps   2        
Notional amounts of interest rate swaps   1,000,000,000        
Floating rate basis LIBOR-based          
Weighted average floating rate 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