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
Pension and Savings Plan
Percentage of eligible earnings used to determine retirement benefits		5.00%		5.00%
Recognized pension expense				$ 33	$ 35	$ 24
Non-qualified pension plan benefit expense				$ 2	3	2
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 on percent of employees' gross pay				6.00%
Savings plan benefit expense				$ 17	16	14
Amounts Recognized in Balance Sheets
Other liabilities-benefit obligations		$ (97)	$ (104)	(97)	(104)
Amounts reclassified from accumulated other comprehensive income: [Abstract]
Tax benefit (expense)				(4)	4	(6)
Net current period other comprehensive income (loss)				4	(6)	8
Other Pension, Postretirement and Supplemental Plans [Member]
Amounts reclassified from accumulated other comprehensive income: [Abstract]
Beginning Balance				3	9
Other comprehensive income (loss) before reclassifications (1)	[1]			7	(10)
Prior service cost (2)	[2]			(1)	0
Tax benefit (expense)				(4)	4
Net current period other comprehensive income (loss)				4	(6)
Ending Balance		7	3	7	3	9
Amounts recognized in accumulated other comprehensive loss
Unrecognized actuarial loss (gain)		(2)	5	(2)	5
Unrecognized prior service cost		6	7	6	7
Total recognized in accumulated other comprehensive loss		4	12	4	12
Less: deferred tax benefit (1)	[3]	(11)	(15)	(11)	(15)
Net amount recognized in accumulated other comprehensive income		$ (7)	$ (3)	(7)	(3)
Changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	[2]			(1)	0
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost — benefits earned during the period				1	1	1
Interest cost on accumulated benefit obligation				5	4	5
Expected return on plan assets				(1)	(1)	(1)
Amortization of prior service cost				1	0	1
Amortization of net loss				0	1	1
Curtailment (1)	[4]			0	(1)	0
Net postretirement benefit cost				$ 6	$ 4	$ 7
Assumptions used to determine net postretirement benefit costs
Discount rate				4.15%	4.35%	3.90%
Expected return on plan assets		3.85%	3.60%	3.60%	3.95%	4.05%
Change in Benefit Obligation [Roll Forward]
Accumulated benefit obligation, beginning of year				$ 115	$ 101
Service cost — benefits earned during the period				1	1	$ 1
Interest cost on accumulated benefit obligation				5	4	5
Benefits paid				(9)	(13)
Participant contributions				3	5
Medicare reimbursement				0	1
Plan amendment (1)	[5]			0	10
Actuarial (gain) loss				(6)	6
Accumulated benefit obligation, end of year		$ 109	$ 115	109	115	101
Change in Plan Assets [Roll Forward]
Plan assets, beginning of year				25	25
Benefits paid				(9)	(13)
Employer contributions				5	7
Participant contributions				3	5
Actual investment return				2	1
Plan assets, end of year		26	25	26	25	$ 25
Amounts Recognized in Balance Sheets
Current liabilities-other		(4)	(4)	(4)	(4)
Other liabilities-benefit obligations		(79)	(86)	(79)	(86)
Net liability, end of year		$ (83)	$ (90)	$ (83)	$ (90)
Discount rate		3.60%	4.15%	3.60%	4.15%
Expected return on plan assets		3.85%	3.60%	3.60%	3.95%	4.05%
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%
Annualized individual discount rates based on time period, minimum				6 months
Annualized individual discount rates based on time period, maximum				99 years
Amounts reclassified from accumulated other comprehensive income: [Abstract]
Prior service cost (2)				$ (1)
Changes in plan assets and benefit obligations recognized in other comprehensive income
Net gain				(7)
Amortization of prior service cost				(1)
Total recognized in other comprehensive income				(8)
Total expense (benefit) recognized in net periodic cost and other comprehensive income				2
Expected amortization of prior service cost for next fiscal year		$ 1		1
Effect of a one-percentage point change in assumed healthcare cost trend rates
Effect of one percentage point increase on postretirement benefit obligation				3
Effect of one percentage point decrease on postretirement benefit obligation				2
Effect of one percentage point increase on total of service and interest cost				0
Effect of one percentage point decrease on total of service and interest cost				$ 0
Minimum [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Amounts Recognized in Balance Sheets
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
Maximum [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Amounts Recognized in Balance Sheets
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
CenterPoint Energy [Member] | Common Stock [Member]
Pension and Savings Plan
Maximum percentage allowed 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] Total other comprehensive income (loss) related to the remeasurement of pension, postretirement and other postemployment plans.

[2] This accumulated other comprehensive component is included in the computation of net periodic cost.

[3] CERC’s postretirement benefit obligation is reduced by the impact of previously non-taxable government subsidies under the Medicare Prescription Drug Act. Because the subsidies were non-taxable, the temporary difference used in measuring the deferred tax impact was determined on the unrecognized losses excluding such subsidies.

[4] Effective January 1, 2017, a change in retiree medical coverage for Medicare eligible post-65 retirees from self-insured to a Medicare Advantage Program, an insured benefit, was implemented. A curtailment gain was recognized in October 2016 related to this implementation.

[5] The postretirement plan was amended in 2016 to change the retiree medical coverage for Medicare eligible post-65 retirees from self-insured to a Medicare Advantage Program, an insured benefit which became effective January 1, 2017.