Derivative Instruments Derivatives and Hedging (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Weather hedges term		10 years
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Aggregate notional amount	[1]	$ 84	$ 84
Not Designated as Hedging Instrument, Economic Hedge [Member] | Houston Electric [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Aggregate notional amount		$ 0	$ 0

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