Long-term Debt (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		0.80%
Other		$ (8.0)		$ (8.0)
Long-term Debt, Excluding Current Maturities		4,489.0		4,680.0
Long-term Debt, Current Maturities	[1]	391.0		372.0
Line of Credit Facility [Abstract]
Size of Credit Facility		300.0		300.0
Annual maturities of long-term debt [Abstract]
2016		0.0
2017		0.0
2018		0.0
2019		200.0
2020		0.0
Transition And System Restoration Bonds [Member]
Annual maturities of long-term debt [Abstract]
2016		391.0
2017		411.0
2018		434.0
2019		458.0
2020		231.0
Bank Loans
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent		200.0		0.0
Long-term Debt, Gross, Current	[1]	0.0		0.0
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
Debt Instrument, Interest Rate, Stated Percentage		9.15%
Annual maturities of long-term debt [Abstract]
Secured Debt		$ 102.0
Replacement Fund Requirements		223.0
Sinking Fund Requirements		1.6
General mortgage bonds 2.25% to 6.95% due 2022 to 2044 (2)
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent	[2]	1,912.0		1,912.0
Long-term Debt, Gross, Current	[1],[2]	$ 0.0		0.0
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum		2.25%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum		6.95%
Annual maturities of long-term debt [Abstract]
Secured Debt		$ 2,100.0
System restoration bonds 3.46% to 4.243% due 2018 to 2022
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent		365.0		415.0
Long-term Debt, Gross, Current	[1]	$ 50.0		48.0
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum		3.46%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum		4.243%
Transition bonds 0.901% to 5.302% due 2017 to 2024
Debt Instrument [Line Items]
Long-term Debt, Gross, Noncurrent		$ 1,918.0		2,259.0
Long-term Debt, Gross, Current	[1]	$ 341.0		324.0
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum		0.901%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum		5.302%
Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Loans Outstanding		$ 200.0	[3]	0.0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		1.637%
Line of Credit [Member]
Line of Credit Facility [Abstract]
Percentage on limitation of debt to total capitalization under covenant (in hundredths)		65.00%
Line of Credit [Member] | CenterPoint Houston [Member]
Line of Credit Facility [Abstract]
Ratio of Indebtedness to Net Capital		0.517
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		1.125%

[1] Includes amounts due or scheduled to be paid within one year of the date noted.

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

[3] Weighted average interest rate was 1.637% as of December 31, 2015.