Stock Based Compensation (Accrual Adjustments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Compensation Cost included in Operation and Maintenance Expense	$ 34	$ 32	$ 32
Income Tax Benefit Recognized in Consolidated Statement of Operations	$ 14	$ 13	$ 13