Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Transition and system restoration property amortization period (in years)	12 to 14 years
Provision for doubtful accounts	$ 1	$ 1	$ 1
Restricted cash	36	40
Prepaid Expenses and Other Current Assets [Member]
Significant Accounting Policies [Line Items]
Restricted cash	35	40
Other Noncurrent Assets [Member]
Significant Accounting Policies [Line Items]
Restricted cash	1	0
Interest Expense [Member]
Significant Accounting Policies [Line Items]
Capitalized interest and AFUDC included in Interest and other finance charges	6	6	8
Other Income [Member]
Significant Accounting Policies [Line Items]
AFUDC equity included in Other Income	$ 10	$ 6	$ 12