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Fair Value Measurements Investments Gains (Losses) (Details) (USD $)
3 Months Ended 6 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Mar. 31, 2012
Investment Gains (Losses)        
Securities classified as trading and measured at fair value $ 1,600,000,000   $ 1,600,000,000  
Equity investments not required to be measured at fair value 2,000,000   2,000,000  
NDT unfunded commitments related to private partnerships 166,000,000   166,000,000  
SERP
       
Investment Gains (Losses)        
Unrealized gains (losses) on investments 1,000,000 2,000,000 1,000,000 3,000,000
NDT
       
Investment Gains (Losses)        
Unrealized gains (losses) on investments 36,000,000 60,000,000 49,000,000 118,000,000
ART
       
Investment Gains (Losses)        
Unrealized gains (losses) on investments $ 14,000,000 $ 15,000,000 $ 17,000,000 $ 25,000,000