Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Tax Act		$ 20	$ (112)
Analysis of Existing Regulatory Liability [Member]
Income Tax Disclosure [Line Items]
Measurement period adjustments		23
Bonus Depreciation [Member]
Income Tax Disclosure [Line Items]
Measurement period adjustments		10
Regulatory Liability [Member]
Income Tax Disclosure [Line Items]
Measurement period adjustments		83
Nondeductible Long-term Incentives [Member]
Income Tax Disclosure [Line Items]
Measurement period adjustments		7
Refundable AMT Credits [Member]
Income Tax Disclosure [Line Items]
Measurement period adjustments	$ 76
Reclassified AMT Credit Carryforwards [Member]
Income Tax Disclosure [Line Items]
Measurement period adjustments		$ 573