Unaudited Quarterly Information (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended									12 Months Ended
	Dec. 31, 2005	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues		$ 585	$ 748	$ 676	$ 531	$ 535	$ 707		$ 606	$ 489	$ 2,540	$ 2,337	$ 2,211
Operating Income		99	242	191	107	93	244		185	101	639	623	573
Income before extraordinary item						18	367	[1]	79	24	279	488	200
Extraordinary item, net of tax						0	598	[1]	0	0
Net Income		27	138	78	36	18	965		79	24	279	1,086	200
Extraordinary Item, net of tax	(947)						598				0	598	0
Return on true-up balance after tax							$ 229

[1] During the third quarter of 2011, CenterPoint Energy recorded an extraordinary gain of $598 million, after-tax, related to the Final Order and a $229 million, after-tax, return on true-up balance included in Income before extraordinary item related to a portion of interest on the appealed amount as discussed in Note 4(b).