Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Tax Act	$ (112)
Impact of tax legislation on transition tax and foreign earnings	10
Income tax benefit related to remeasurement of deferred tax asset	534
Regulatory Liabilities	15,732	$ 7,290
Tax Act [Member]
Income Tax Disclosure [Line Items]
Regulatory Liabilities	8,313
Valuation Allowance, Tax Credit Carryforward [Member]
Income Tax Disclosure [Line Items]
Tax Act	$ 406