Related Party Transactions and Major Customers (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Money Pool Borrowings	$ 60								$ 60
Investments in Money Pool					$ 96					$ 96
Weighted average interest rate	1.90%								1.90%
Affiliate related net interest income (expense)									$ 2	(4)	$ (1)
Cash dividends paid to parent company									180	135	252
Proceeds from contributions from parent									0	374	0
Revenues	$ 765	$ 843	$ 752	$ 638	$ 731	$ 909	$ 763	$ 656	2,998	3,059	2,846
Affiliates of NRG Energy, Inc. [Member]
Related Party Transaction [Line Items]
Revenues									713	698	741
Affiliates of Vistra Energy Corp. [Member]
Related Party Transaction [Line Items]
Revenues									229	220	220
Operation And Maintenance Expense [Member] | CenterPoint Energy [Member]
Related Party Transaction [Line Items]
Corporate service charges									188	179	176
Operation And Maintenance Expense [Member] | CERC Corp [Member]
Related Party Transaction [Line Items]
Corporate service charges									8	7	6
Other revenues from transactions with related party									$ (17)	$ (15)	$ (18)