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Debt (Details 2) (USD $)
6 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Jun. 30, 2013
Fixed rate senior notes
Dec. 31, 2012
Fixed rate senior notes
Jun. 30, 2013
Credit agreements
Jun. 30, 2012
Credit agreements
Debt            
Debt instrument face value     $ 6,300,000,000 $ 6,300,000,000    
Debt instrument fair value     6,800,000,000 7,300,000,000    
Borrowings under credit agreements         7,561,000,000 4,856,000,000
Repayments under credit agreements         7,747,000,000 4,511,000,000
Outstanding letters of credit $ 50,000,000 $ 24,000,000