Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019
Lease, Cost [Abstract]
Operating lease cost	$ 9	$ 7
Short-term lease cost	14	25
Total lease cost	23	32
Operating Leases, Lease Income [Abstract]
Operating lease income	5	4
Variable lease income	1	2
Total lease income	6	6
Assets and Liabilities, Lessee [Abstract]
Operating ROU assets (1)	31	31
Total leased assets	$ 31	$ 31	$ 22
Weighted-average remaining lease term (in years) - operating leases	6 years	6 years 6 months
Weighted-average discount rate - operating leases	3.14%	3.57%
Operating Lease Liabilities, Payments Due [Abstract]
2021	$ 8
2022	6
2023	6
2024	4
2025	3
2026 and beyond	10
Total lease payments	37
Less: Interest	5
Lessor, Operating Lease, Payments, Fiscal Year Maturity [Abstract]
2021	4
2022	2
2023	2
2024	2
2025	2
2026 and beyond	8
Total lease payments to be received	20
Other Information Related to Leases [Abstract]
Operating cash flows from operating leases included in the measurement of lease liabilities	8
Lessee Operating Lease Not Yet Commenced	847
Other Current Liabilities [Member]
Assets and Liabilities, Lessee [Abstract]
Current operating lease liability (2)	6	$ 7
Other Liabilities
Assets and Liabilities, Lessee [Abstract]
Total leased liabilities	32	31
Other Noncurrent Liabilities [Member]
Assets and Liabilities, Lessee [Abstract]
Non-current operating lease liability (3)	26	24
Houston Electric [Member]
Lease, Cost [Abstract]
Operating lease cost	0	0
Short-term lease cost	12	23
Total lease cost	12	23
Operating Leases, Lease Income [Abstract]
Operating lease income	0	2
Variable lease income	0	0
Total lease income	0	2
Assets and Liabilities, Lessee [Abstract]
Operating ROU assets (1)	1	1
Total leased assets	1	1	1
Current operating lease liability (2)	0	0
Non-current operating lease liability (3)	$ 1	$ 1
Weighted-average remaining lease term (in years) - operating leases	4 years	5 years 2 months 12 days
Weighted-average discount rate - operating leases	2.59%	3.52%
Operating Lease Liabilities, Payments Due [Abstract]
2021	$ 1
2022	0
2023	0
2024	0
2025	0
2026 and beyond	0
Total lease payments	1
Less: Interest	0
Lessor, Operating Lease, Payments, Fiscal Year Maturity [Abstract]
2021	0
2022	0
2023	0
2024	0
2025	0
2026 and beyond	0
Total lease payments to be received	0
Other Information Related to Leases [Abstract]
Operating cash flows from operating leases included in the measurement of lease liabilities	1
Houston Electric [Member] | Other Liabilities
Assets and Liabilities, Lessee [Abstract]
Total leased liabilities	1	$ 1
CERC Corp [Member]
Lease, Cost [Abstract]
Operating lease cost	5	4
Short-term lease cost	0	0
Total lease cost	5	4
Operating Leases, Lease Income [Abstract]
Operating lease income	2	1
Variable lease income	0	0
Total lease income	2	1
Assets and Liabilities, Lessee [Abstract]
Operating ROU assets (1)	19	18
Total leased assets	19	18	$ 19
Current operating lease liability (2)	3	3
Non-current operating lease liability (3)	$ 18	$ 15
Weighted-average remaining lease term (in years) - operating leases	7 years 6 months	7 years 1 month 6 days
Weighted-average discount rate - operating leases	3.36%	3.61%
Operating Lease Liabilities, Payments Due [Abstract]
2021	$ 4
2022	4
2023	4
2024	3
2025	2
2026 and beyond	7
Total lease payments	24
Less: Interest	3
Lessor, Operating Lease, Payments, Fiscal Year Maturity [Abstract]
2021	1
2022	0
2023	0
2024	0
2025	0
2026 and beyond	0
Total lease payments to be received	1
Other Information Related to Leases [Abstract]
Operating cash flows from operating leases included in the measurement of lease liabilities	4
CERC Corp [Member] | Other Liabilities
Assets and Liabilities, Lessee [Abstract]
Total leased liabilities	$ 21	$ 18