Revenue Recognition (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Bad debt expense	$ 24	$ 18	$ 16
Accounting Standards Update 2016-13 [Member]
Cumulative Effect on Retained Earnings	5
Accounting Standards Update 2016-13 [Member] | Energy Services [Member]
Cumulative Effect on Retained Earnings	2
Houston Electric [Member]
Bad debt expense	0	0	0
CERC Corp [Member]
Bad debt expense	18	$ 14	$ 16
CERC Corp [Member] | Accounting Standards Update 2016-13 [Member]
Cumulative Effect on Retained Earnings	$ 5