Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
LIFO Inventory Amount	$ 92	$ 97
Excess of Replacement or Current Costs over Stated LIFO Value	62
Interest Expense [Member]
Significant Accounting Policies [Line Items]
Interest and AFUDC debt (1)	27	36	$ 8
Other Income (Expense) [Member]
Significant Accounting Policies [Line Items]
AFUDC equity (2)	25	22	12
Houston Electric [Member] | Interest Expense [Member]
Significant Accounting Policies [Line Items]
Interest and AFUDC debt (1)	8	8	6
Houston Electric [Member] | Other Income (Expense) [Member]
Significant Accounting Policies [Line Items]
AFUDC equity (2)	14	15	10
CERC Corp [Member]
Significant Accounting Policies [Line Items]
LIFO Inventory Amount	55	58
Excess of Replacement or Current Costs over Stated LIFO Value		54
CERC Corp [Member] | Interest Expense [Member]
Significant Accounting Policies [Line Items]
Interest and AFUDC debt (1)	3	3	2
CERC Corp [Member] | Other Income (Expense) [Member]
Significant Accounting Policies [Line Items]
AFUDC equity (2)	$ 3	$ 3	$ 2