Related Party Transactions and Major Customers (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Related Party Transaction [Line Items]
Money Pool Borrowings	$ (312)								$ (312)
Investments in Money Pool					$ 107					$ 107
Debt, Weighted Average Interest Rate	0.80%								0.80%
Interest Income (Expense) Net, Related Party									$ (1)	(1)	$ 20
Cash Dividends Paid to Parent Company									252	0	766
Proceeds from Contributions from Parent									0	90	0
Revenues	$ 697	$ 827	$ 705	$ 617	$ 679	$ 839	$ 698	$ 630	2,846	2,846	2,577
Affiliates of NRG Energy, Inc. [Member]
Related Party Transaction [Line Items]
Revenues									741	735	658
Affiliates of Energy Future Holdings Corp. [Member]
Related Party Transaction [Line Items]
Revenues									220	189	167
Operation And Maintenance Expense Member | CenterPoint Energy [Member]
Related Party Transaction [Line Items]
Corporate service charges									176	159	147
Operation And Maintenance Expense Member | CERC Corp [Member]
Related Party Transaction [Line Items]
Corporate service charges									6	5	9
Related Party Transaction, Other Revenues from Transactions with Related Party									$ (18)	$ (17)	$ (21)