Reportable Segments - Revenues From Major External Customers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 8,352	$ 7,418	$ 7,564
Houston Electric
Segment Reporting Information [Line Items]
Total revenues	3,134	2,911	2,990
Affiliates of NRG | Houston Electric
Segment Reporting Information [Line Items]
Total revenues	905	749	727
Affiliates of Vistra Energy Corp. | Houston Electric
Segment Reporting Information [Line Items]
Total revenues	$ 410	$ 404	$ 263