Long-term Debt (Details) $ in Millions			12 Months Ended
		Jan. 12, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 02, 2018 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Unamortized debt issuance costs			$ (22.0)		$ (23.0)
Unamortized discount and premium, net			(10.0)		(9.0)
Long-term Debt, Excluding Current Maturities			4,319.0		4,454.0
Long-term Debt, Current Maturities	[1]		434.0		411.0
Line of Credit Facility [Abstract]
Size of Credit Facility			300.0		300.0
Annual maturities of long-term debt [Abstract]
2018	[2]		434.0
2019	[2]		458.0
2020	[2]		231.0
2021	[2]		613.0
2022	[2]		519.0
First mortgage bonds 9.15% due 2021
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent			102.0		102.0
Long-term Debt, Gross, Current	[1]		$ 0.0		0.0
Interest rate			9.15%
Annual maturities of long-term debt [Abstract]
Secured debt			$ 102.0
Replacement fund requirements			266.0
Sinking fund requirements			1.6
General mortgage bonds 1.85% to 6.95% due 2021 to 2044 (2)
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent	[3]		2,812.0		2,512.0
Long-term Debt, Gross, Current	[1],[3]		0.0		0.0
Annual maturities of long-term debt [Abstract]
Secured debt			2,900.0
System restoration bonds 3.46% to 4.243% due 2018 to 2022
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent			256.0		312.0
Long-term Debt, Gross, Current	[1]		56.0		53.0
Transition bonds 2.161% to 5.302% due 2019 to 2024
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent			1,181.0		1,560.0
Long-term Debt, Gross, Current	[1]		378.0		358.0
Bonds General Mortgage Due 2027 [Member]
Debt Instrument [Line Items]
Aggregate principal amount		$ 300.0
Interest rate		3.00%
Maturity date		Feb. 01, 2027
Transition And System Restoration Bonds [Member]
Annual maturities of long-term debt [Abstract]
2018			434.0
2019			458.0
2020			231.0
2021			211.0
2022			219.0
Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Loans Outstanding			$ 0.0		0.0
Line of Credit [Member]
Line of Credit Facility [Abstract]
Percentage on limitation of debt to total capitalization under covenant (in hundredths)	[4]		65.00%
Ratio of indebtedness to net capital	[5]		0.486
Percentage on limitation of debt to total capitalization under covenant amended (in hundredths)			70.00%
Restoration cost expected cost			$ 100.0
Consecutive period for system restoration costs to exceed $100 million (in months)			12
Letter of Credit [Member]
Line of Credit Facility [Abstract]
Loans Outstanding			$ 4.0		$ 4.0
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit [Member]
Line of Credit Facility [Abstract]
Line of credit facility interest rate description	[6]		1.125%
January thru February [Member] | Treasury Lock [Member] | Subsequent Event [Member]
Debt Instrument [Line Items]
Aggregate notional amount				$ 200.0
January [Member] | Treasury Lock [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 300.0
Aggregate notional amount			150.0
Effective portion of realized losses			$ 1.0
Minimum [Member] | General mortgage bonds 1.85% to 6.95% due 2021 to 2044 (2)
Debt Instrument [Line Items]
Interest rate			1.85%
Minimum [Member] | System restoration bonds 3.46% to 4.243% due 2018 to 2022
Debt Instrument [Line Items]
Interest rate			3.46%
Minimum [Member] | Transition bonds 2.161% to 5.302% due 2019 to 2024
Debt Instrument [Line Items]
Interest rate			2.161%
Maximum [Member] | General mortgage bonds 1.85% to 6.95% due 2021 to 2044 (2)
Debt Instrument [Line Items]
Interest rate			6.95%
Maximum [Member] | System restoration bonds 3.46% to 4.243% due 2018 to 2022
Debt Instrument [Line Items]
Interest rate			4.243%
Maximum [Member] | Transition bonds 2.161% to 5.302% due 2019 to 2024
Debt Instrument [Line Items]
Interest rate			5.302%

[1] Includes amounts due or exchangeable within one year of the date noted.

[2] These maturities include Securitization Bonds principal repayments on scheduled payment dates.

[3] Debt issued as collateral is excluded from the financial statements because of the contingent nature of the obligation.

[4] 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.

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

[6] Based on current credit ratings.