Revenue Recognition and Allowance for Credit Losses - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue from contracts	$ 1,715	$ 1,566	$ 4,256	$ 3,707
Other	27	9	33	35
Revenues	1,742	1,575	4,289	3,742
Lease Income	2	1	4	2
Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts	933	850	1,766	1,617
Other	4	(2)	1	(2)
Revenues	937	848	1,767	1,615
Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts	718	634	2,373	1,930
Other	22	10	30	35
Revenues	740	644	2,403	1,965
Corporate and Other
Disaggregation of Revenue [Line Items]
Revenue from contracts	64	82	117	160
Other	1	1	2	2
Revenues	65	83	119	162
Houston Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts	791	722	1,478	1,360
Other	(5)	(2)	(8)	(6)
Revenues	786	720	1,470	1,354
CERC Corp
Disaggregation of Revenue [Line Items]
Revenue from contracts	558	472	1,730	1,456
Other	16	11	21	38
Revenues	$ 574	$ 483	$ 1,751	$ 1,494