Related Party Transactions and Major Customers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Money Pool Borrowings	$ (120)		$ (120)
Investments in Money Pool					$ 107
Corporate service charges	40	$ 41	126	$ 117
Sales to major customers	827	839	2,149	2,167
Affiliates of NRG Energy, Inc. [Member]
Related Party Transaction [Line Items]
Sales to major customers	222	222	578	552
Affiliates of Energy Future Holdings Corp. [Member]
Related Party Transaction [Line Items]
Sales to major customers	$ 67	$ 59	$ 170	$ 140