Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Reclassification of ATM credit carryforwards	$ 573
Provisional adjustments for TCJA	3	$ 76	$ 73
Regulatory liability representing revaluation of deferred tax balances	$ 57

Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense

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