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Debt (Details 2) (USD $)
Mar. 31, 2013
Dec. 31, 2012
Debt    
Outstanding letters of credit $ 56,000,000 $ 24,000,000
Fixed rate senior notes
   
Debt    
Debt instrument face value 6,300,000,000 6,300,000,000
Debt instrument fair value $ 7,200,000,000 $ 7,300,000,000