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Investments (Recorded Other-Than-Temporary Impairment Losses on Investments) (Details) - USD ($)
 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]        
Total other-than-temporary impairment losses [1] $ 7,000,000 $ 9,000,000 $ 33,000,000 $ 34,000,000
Losses recorded to the nuclear decommissioning trust regulatory liability (2,000,000) (4,000,000) (13,000,000) (15,000,000)
Losses recognized in other comprehensive income (before taxes) (1,000,000)   (2,000,000) (1,000,000)
Net impairment losses recognized in earnings 4,000,000 5,000,000 18,000,000 18,000,000
Maximum        
Schedule of Available-for-sale Securities [Line Items]        
Other-than-temporary impairment losses for debt securities $ 1,000,000 $ 1,000,000 $ 2,000,000 $ 2,000,000
[1] Amounts include other-than-temporary impairment losses for debt securities of less than $1 million for both the three months ended September 2017 and 2016, respectively, and $2 million for both the nine months ended September 30, 2017 and 2016, respectively.