Long-term Debt (Details) $ in Millions			3 Months Ended
		Feb. 28, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Size of credit facility			$ 300	$ 300
General mortgage bonds used as collateral			118	118
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term line of credit			0	0
Letter of Credit [Member]
Debt Instrument [Line Items]
Long-term line of credit			$ 4	$ 4
Line of Credit [Member]
Debt Instrument [Line Items]
Percentage on limitation of debt to total capitalization under covenant	[1]		65.00%
Percentage on limitation of debt to total capitalization under covenant amended (in hundredths)			70.00%
Ratio of indebtedness to net capital	[2]		0.512
System restoration costs threshold for increase in permitted debt to EBITDA covenant ratio			$ 100
Consecutive period for system restoration costs to exceed $100 million (in months)			12
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on LIBOR	[3]		1.125%
Treasury Lock [Member] | January thru February [Member]
Debt Instrument [Line Items]
Aggregate notional amount			$ 200
Effective portion of realized gains			$ 5
General Mortgage Bonds Due 2048 [Member]
Debt Instrument [Line Items]
Principal amount of debt issued		$ 400
Interest rate of debt issued		3.95%
Maturity date of debt issued		Mar. 01, 2048

[1] The financial covenant limit will temporarily increase from 65% to 70% if Houston Electric experiences damage from a natural disaster in its service territory and Houston Electric certifies to the administrative agent that Houston Electric has incurred system restoration costs reasonably likely to exceed $100 million in a consecutive 12-month period, all or part of which Houston Electric intends to seek to recover through securitization financing. Such temporary increase in the financial covenant would be in effect from the date Houston Electric delivers its certification until the earliest to occur of (i) the completion of the securitization financing, (ii) the first anniversary of Houston Electric’s certification or (iii) the revocation of such certification.

[2] As defined in the revolving credit facility agreement, excluding Securitization Bonds.

[3] Based on current credit ratings.