Employee Benefit Plans Pension and Postretirement Benefits (Details) - USD ($) $ in Millions				12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Percentage of eligible earnings used to determine retirement benefits		5.00%		5.00%
Recognized pension expense				$ 41	$ 44	$ 35
Non-qualified pension plan expense				$ 1	1	1
Maximum percentage of employee compensation eligible for contribution to savings plan				50.00%
Description of company match under savings plan				100.00%
Employer matching contribution percentage of employee's gross pay				6.00%
Savings plan benefit expense				$ 17	15	$ 14
Components of net periodic costs [Abstract]
Service cost				1	1
Interest cost				$ 9	$ 10
Assumptions used to determine net periodic benefit (income) cost [Abstract]
Discount rate				4.15%	4.35%	3.90%
Expected long-term return on assets		4.75%	4.75%	4.75%	5.00%	5.45%
Change in Benefit Obligation
Accumulated benefit obligation, beginning of year				$ 217	$ 283
Service cost				1	1
Interest cost				9	10
Benefits paid				(14)	(20)
Participant contributions				2	3
Medicare drug reimbursement				0	1
Plan amendment (1)	[1]			0	(65)
Actuarial loss				10	4
Accumulated benefit obligation, end of year		$ 225	$ 217	225	217	$ 283
Change in Plan Assets
Plan assets, beginning of year				88	110
Benefits paid				(14)	(20)
Employer contributions				10	10
Participant contributions				2	3
Actual investment return				7	5
Plan amendment (2)	[2]			0	(20)
Plan assets, end of year		93	88	93	88	$ 110
Amounts Recognized in Balance Sheets
Other liabilities-benefit obligations		$ (146)	$ (148)	$ (146)	$ (148)
Actuarial Assumptions
Discount rate		3.60%	4.15%	3.60%	4.15%
Expected long-term return on assets		4.75%	4.75%	4.75%	5.00%	5.45%
Prescription drug cost trend rate assumed for the next year - Pre-65		9.85%			10.75%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)		4.50%	4.50%	4.50%	4.50%
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Effect of One Percentage Point Increase on Postretirement Benefit Obligation				$ 9
Effect of One Percentage Point Decrease on Postretirement Benefit Obligation				8
Effect of One Percentage Point Increase on Total Service and Interest Cost				0
Effect of One Percentage Point Decrease on Total Service and Interest Cost				$ 0
Annualized individual discount rates based on time period, minimum				6 months
Annualized individual discount rates based on time period, maximum				99 years
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic costs [Abstract]
Service cost				$ 1	$ 1	$ 1
Interest cost				9	10	13
Expected return on plan assets				(4)	(5)	(6)
Amortization of prior service credit				(6)	(4)	(2)
Amortization of loss				0	1	3
Curtailment				0	(4)	0
Net postretirement benefit cost (credit)				0	(1)	9
Change in Benefit Obligation
Service cost				1	1	1
Interest cost				9	10	$ 13
Amounts Recognized in Balance Sheets
Other liabilities-benefit obligations		$ (132)	$ (129)	(132)	(129)
Net liability, end of year		$ (132)	$ (129)	$ (132)	$ (129)
Maximum [Member]
Actuarial Assumptions
Healthcare cost trend rate assumed for the next year		23.85%	10.65%	23.85%	10.65%
Year that the cost trend rates reach the ultimate trend rate				2024	2024
Minimum [Member]
Actuarial Assumptions
Healthcare cost trend rate assumed for the next year		6.15%	5.75%	6.15%	5.75%
Prescription drug cost trend rate assumed for the next year - Pre-65		9.85%
Year that the cost trend rates reach the ultimate trend rate				2026	2024
CenterPoint Energy [Member] | Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of account balance in company stock				25.00%
Number of shares in common stock held by savings plan		12,806,085		12,806,085
Percentage of investment in common stocks		16.00%		16.00%

[1] The postretirement benefits were amended during 2016 to change retiree medical coverage, effective January 1, 2017, as follows: (i) members of the IBEW Local Union 66 who retire on or after January 1, 2017, and their dependents, will receive any retiree medical and prescription drug coverage exclusively through the NECA/IBEW Family Medical Care Plan pursuant to the terms of the renegotiated collective bargaining agreement entered into in May 2016; and (ii) Medicare eligible post-65 retirees will receive coverage through a Medicare Advantage Program, an insured benefit, in lieu of the previous self-insured benefit. These changes resulted in a reduction in our postretirement plan liability of $65 million as of December 31, 2016.

[2] In May 2016, Houston Electric entered into a renegotiated collective bargaining agreement with the IBEW Local Union 66. The Houston Lighting & Power Company Union Retirees’ Medical and Dental Benefits Trust was amended to reflect the renegotiated collective bargaining agreement by establishing a segregated and restricted account under the trust for the retiree medical benefits of post-2016 union retirees who are now covered exclusively by the NECA/IBEW Family Medical Care Plan. $20 million was transferred to the account for post-2016 union retirees.