New Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accounting Standards Update 2016-15 [Member]
Effect of change on cash flow activities due to adoption of new ASU			$ 1	$ 3
Accounting Standards Update 2016-18 [Member]
Effect of change on cash flow activities due to adoption of new ASU			(2)	8
Accounting Standards Update 2017-07 [Member] | Operating Income [Member]
Increase to operating income and corresponding (decrease) to other income from the adoption of ASU 2017-07	$ 15	$ 17	29	34
Accounting Standards Update 2017-07 [Member] | Other Income [Member]
Increase to operating income and corresponding (decrease) to other income from the adoption of ASU 2017-07	(15)	(17)	(29)	(34)
Accounting Standards Update 2017-07 [Member] | Houston Electric [Member] | Operating Income [Member]
Increase to operating income and corresponding (decrease) to other income from the adoption of ASU 2017-07	8	7	15	15
Accounting Standards Update 2017-07 [Member] | Houston Electric [Member] | Other Income [Member]
Increase to operating income and corresponding (decrease) to other income from the adoption of ASU 2017-07	(8)	(7)	(15)	(15)
Accounting Standards Update 2017-07 [Member] | CERC Corp [Member] | Operating Income [Member]
Increase to operating income and corresponding (decrease) to other income from the adoption of ASU 2017-07	4	6	8	11
Accounting Standards Update 2017-07 [Member] | CERC Corp [Member] | Other Income [Member]
Increase to operating income and corresponding (decrease) to other income from the adoption of ASU 2017-07	$ (4)	$ (6)	$ (8)	$ (11)