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Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Available-for-sale securities:    
Proceeds from sales $ 415 $ 502
Realized gains 28 [1] 14 [1]
Realized losses $ 4 [1] $ 8 [1]
[1] Includes realized gains or losses recorded to the decommissioning trust regulatory liability.