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Debt (Details) (USD $)
3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2012
Notes 4.70% [Member]
Mar. 31, 2012
Notes 4.75% [Member]
Debt Instrument [Line Items]        
Fair value of long-term debt $ 3,400,000,000 $ 2,600,000,000    
Par value of issued senior unsecured notes     250,000,000 500,000,000
Interest rate on debt     4.70% 4.75%
Proceeds from issuance of long-term debt and capital securities, net     $ 247,000,000 $ 495,000,000
Maturity date     Mar. 15, 2037 Mar. 15, 2042