Derivative Instruments Derivatives and Hedging (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Weather hedges term				10
Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Realized losses deferred to other comprehensive income	[1]	$ 0	$ 2	$ 0	$ 3
Houston Electric [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Realized losses deferred to other comprehensive income	[1]	0	0	0	1
CERC Corp [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Realized losses deferred to other comprehensive income	[1]	0	$ 0	0	$ 0
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Aggregate notional amount	[2]	$ 84		$ 84		$ 84

[1] Gains and losses are reclassified from Accumulated other comprehensive income into income when the hedged transactions affect earnings. The reclassification amounts are included in Interest and other finance charges in each of the Registrants’ respective Statements of Consolidated Income. Over the next twelve months estimated amortization from Accumulated Comprehensive Income into income is expected to be immaterial.

[2] Relates to interest rate derivative instruments at SIGECO.