Stock Based Compensation (Accrual Adjustments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Compensation Cost included in Operation and Maintenance Expense	$ 31	$ 29	$ 34
Income Tax Benefit Recognized in Consolidated Statement of Operations	13	12	14
Excess Tax Benefits	$ 4	$ 4	$ 3