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Fair Value (Fair Value of Fixed Income Securities Held in Nuclear Decommissioning Trust Funds (Details) - Nuclear decommissioning trusts - Fixed Income Securities
$ in Millions
Dec. 31, 2017
USD ($)
Schedule of Available-for-sale Securities [Line Items]  
Due within one year $ 23
Due after one through five years 102
Due after five through ten years 115
Due after ten years 255
Fixed income securities total $ 495