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Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Schedule Of Available For Sale Securities [Line Items]        
Proceeds from sales $ 377 $ 300 $ 1,496 $ 1,009
Realized gains [1] 25 40 142 102
Realized losses [1] 16 9 52 43
Virginia Electric and Power Company        
Schedule Of Available For Sale Securities [Line Items]        
Proceeds from sales 156 131 654 478
Realized gains [1] 9 18 64 48
Realized losses [1] $ 6 $ 4 $ 24 $ 21
[1] Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability.