Related Party Transactions and Major Customers (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Money Pool Borrowings	$ (131)		$ (131)
Investments in Money Pool					$ 107
Corporate service charges	44	$ 39	86	$ 76
Sales to major customers	705	698	1,322	1,328
Affiliates of NRG Energy, Inc. [Member]
Related Party Transaction [Line Items]
Sales to major customers	172	164	356	330
Affiliates of Energy Future Holdings Corp. [Member]
Related Party Transaction [Line Items]
Sales to major customers	$ 51	$ 41	$ 103	$ 81