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Fair Value (Fair Value of Fixed Income Securities Held in Nuclear Decommissioning Trust Funds (Details) - Fixed income securities - Nuclear decommissioning trust fund
$ in Millions
Mar. 31, 2018
USD ($)
Schedule of Available-for-sale Securities [Line Items]  
Due within one year $ 25
Due after one through five years 108
Due after five through ten years 117
Due after ten years 280
Available-for-sale securities total $ 530