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Fair Value Measurements (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Assets      
Natural gas derivatives (3)(4) $ 57 $ 80  
Total derivative assets, netting adjustment [1] (82) (64)  
Total assets 747 1,199  
Liabilities      
Total derivative liabilities, netting adjustment [1] (101) (83)  
Total liabilities 732 692  
Fair value measurement using significant unobservable input (level 3) reconciliation [Roll Forward]      
Beginning balance (622) [2] (704) [2] $ 12
Purchases (1) [3] 0 0 12
Total gains (losses) 30 96 12
Total settlements (39) (11) (27)
Transfers into Level 3 (2) [4] 5 14 (712)
Transfers out of Level 3 (3) [5] 656 (17) (1)
Ending balance (4) [2] 30 (622) (704)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at the reporting date (3) $ 18 87 (402)
Atmos Energy Marketing [Member]      
Fair value measurement using significant unobservable input (level 3) reconciliation [Roll Forward]      
Mark-to-market value of Level 3 derivative assets acquired less than $1 million    
Carrying amount [Member]      
Estimated Fair Value of Financial Instruments      
Long-term debt, including current maturities [6] $ 9,140 8,679  
Fair value [Member]      
Estimated Fair Value of Financial Instruments      
Long-term debt, including current maturities [6] 9,308 9,220  
Natural gas [Member]      
Liabilities      
Derivative fair value offsets, net 19 19  
Fair Value, Inputs, Level 1 [Member]      
Assets      
Total assets [7] 609 1,045  
Liabilities      
Total liabilities [7] 24 0  
Fair Value, Inputs, Level 2 [Member]      
Assets      
Total assets 173 161  
Liabilities      
Total liabilities 792 96  
Fair Value, Inputs, Level 3 [Member]      
Assets      
Total assets 47 57  
Liabilities      
Total liabilities $ 17 679  
Fair Value, Inputs, Level 3 [Member] | Forward Contracts [Member] | Minimum [Member]      
Fair Value, assets and liabilities measured on recurring basis, financial statement captions      
Fair value inputs (price per MMBtu) 1.39    
Fair Value, Inputs, Level 3 [Member] | Forward Contracts [Member] | Maximum [Member]      
Fair Value, assets and liabilities measured on recurring basis, financial statement captions      
Fair value inputs (price per MMBtu) 5.96    
Fair Value, Measurements, Recurring [Member]      
Assets      
Corporate equities $ 542 963  
Investments, including money market funds (2) [8] 66 68  
Hedged portion of natural gas inventory 1 14  
Fair Value, Measurements, Recurring [Member] | Natural gas [Member]      
Assets      
Natural gas derivative netting adjustment [1],[9] (82) (64)  
Natural gas derivatives (3)(4) [9] 138 154  
Liabilities      
Derivative liabilities, netting adjustment [1],[9] (101) (83)  
Derivative liabilities [9] 107 24  
Fair Value, Measurements, Recurring [Member] | IDS Derivative [Member]      
Liabilities      
Derivative liabilities, netting adjustment [1] 0 0  
Derivative liabilities 601 668  
Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities 24 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Assets      
Corporate equities [7] 542 963  
Investments, including money market funds (2) [7],[8] 66 68  
Hedged portion of natural gas inventory [7] 1 14  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Natural gas [Member]      
Assets      
Natural gas derivatives (3)(4) [7],[9] 0 0  
Liabilities      
Derivative liabilities [7],[9] 0 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | IDS Derivative [Member]      
Liabilities      
Derivative liabilities [7] 0 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities [7] 24 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]      
Assets      
Corporate equities 0 0  
Investments, including money market funds (2) [8] 0 0  
Hedged portion of natural gas inventory 0 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Natural gas [Member]      
Assets      
Natural gas derivatives (3)(4) [9] 173 161  
Liabilities      
Derivative liabilities [9] 191 96  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | IDS Derivative [Member]      
Liabilities      
Derivative liabilities 601 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities 0 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Assets      
Corporate equities 0 0  
Investments, including money market funds (2) [8] 0 0  
Hedged portion of natural gas inventory 0 0  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Natural gas [Member]      
Assets      
Natural gas derivatives (3)(4) [9] 47 57  
Liabilities      
Derivative liabilities [9] 17 11  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | IDS Derivative [Member]      
Liabilities      
Derivative liabilities 0 668  
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities 0 0  
Houston Electric [Member]      
Assets      
Total derivative assets, netting adjustment 0 0  
Total assets 48 51  
Liabilities      
Total derivative liabilities, netting adjustment 0 0  
Total liabilities 24 0  
Houston Electric [Member] | Carrying amount [Member]      
Estimated Fair Value of Financial Instruments      
Long-term debt, including current maturities [6] 4,717 4,753  
Houston Electric [Member] | Fair value [Member]      
Estimated Fair Value of Financial Instruments      
Long-term debt, including current maturities [6] 4,770 5,034  
Houston Electric [Member] | Fair Value, Inputs, Level 1 [Member]      
Assets      
Total assets 48 51  
Liabilities      
Total liabilities 24 0  
Houston Electric [Member] | Fair Value, Inputs, Level 2 [Member]      
Assets      
Total assets 0 0  
Liabilities      
Total liabilities 0 0  
Houston Electric [Member] | Fair Value, Inputs, Level 3 [Member]      
Assets      
Total assets 0 0  
Liabilities      
Total liabilities 0 0  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member]      
Assets      
Investments, including money market funds (2) [8] 48 51  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities 24 0  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Assets      
Investments, including money market funds (2) [8] 48 51  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities 24 0  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]      
Assets      
Investments, including money market funds (2) [8] 0 0  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities 0 0  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Assets      
Investments, including money market funds (2) [8] 0 0  
Houston Electric [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]      
Liabilities      
Derivative liabilities 0 0  
CERC Corp [Member]      
Assets      
Natural gas derivatives (3)(4) 57 80  
Total derivative assets, netting adjustment [1] (82) (64)  
Total assets 152 182  
Liabilities      
Total derivative liabilities, netting adjustment [1] (101) (83)  
Total liabilities 107 24  
Fair value measurement using significant unobservable input (level 3) reconciliation [Roll Forward]      
Beginning balance 46 [2] 13 [2] 12
Purchases (1) [3] 0 0 12
Total gains (losses) 30 47 12
Total settlements (39) (11) (27)
Transfers into Level 3 (2) [4] 5 14 5
Transfers out of Level 3 (3) [5] (12) (17) (1)
Ending balance (4) [2] 30 46 13
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at the reporting date (3) 18 38 $ 11
CERC Corp [Member] | Carrying amount [Member]      
Estimated Fair Value of Financial Instruments      
Long-term debt, including current maturities 2,371 2,457  
CERC Corp [Member] | Fair value [Member]      
Estimated Fair Value of Financial Instruments      
Long-term debt, including current maturities 2,488 2,708  
CERC Corp [Member] | Natural gas [Member]      
Liabilities      
Derivative fair value offsets, net 19 19  
CERC Corp [Member] | Fair Value, Inputs, Level 1 [Member]      
Assets      
Total assets [7] 14 28  
Liabilities      
Total liabilities [7] 0 0  
CERC Corp [Member] | Fair Value, Inputs, Level 2 [Member]      
Assets      
Total assets 173 161  
Liabilities      
Total liabilities 191 96  
CERC Corp [Member] | Fair Value, Inputs, Level 3 [Member]      
Assets      
Total assets 47 57  
Liabilities      
Total liabilities $ 17 11  
CERC Corp [Member] | Fair Value, Inputs, Level 3 [Member] | Forward Contracts [Member] | Minimum [Member]      
Fair Value, assets and liabilities measured on recurring basis, financial statement captions      
Fair value inputs (price per MMBtu) 1.39    
CERC Corp [Member] | Fair Value, Inputs, Level 3 [Member] | Forward Contracts [Member] | Maximum [Member]      
Fair Value, assets and liabilities measured on recurring basis, financial statement captions      
Fair value inputs (price per MMBtu) 5.96    
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member]      
Assets      
Corporate equities $ 2 3  
Investments, including money market funds (2) [8] 11 11  
Hedged portion of natural gas inventory 1 14  
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member] | Natural gas [Member]      
Assets      
Natural gas derivative netting adjustment [1],[9] (82) (64)  
Natural gas derivatives (3)(4) [9] 138 154  
Liabilities      
Derivative liabilities, netting adjustment [1],[9] (101) (83)  
Derivative liabilities [9] 107 24  
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]      
Assets      
Corporate equities [7] 2 3  
Investments, including money market funds (2) [7],[8] 11 11  
Hedged portion of natural gas inventory [7] 1 14  
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Natural gas [Member]      
Assets      
Natural gas derivatives (3)(4) [7],[9] 0 0  
Liabilities      
Derivative liabilities [7],[9] 0 0  
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]      
Assets      
Corporate equities 0 0  
Investments, including money market funds (2) [8] 0 0  
Hedged portion of natural gas inventory 0 0  
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Natural gas [Member]      
Assets      
Natural gas derivatives (3)(4) [9] 173 161  
Liabilities      
Derivative liabilities [9] 191 96  
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Assets      
Corporate equities 0 0  
Investments, including money market funds (2) [8] 0 0  
Hedged portion of natural gas inventory 0 0  
CERC Corp [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Natural gas [Member]      
Assets      
Natural gas derivatives (3)(4) [9] 47 57  
Liabilities      
Derivative liabilities [9] $ 17 $ 11  
[1] Amounts represent the impact of legally enforceable master netting arrangements that allow CenterPoint Energy and CERC to settle positive and negative positions and also include cash collateral of $19 million as of both December 31, 2018 and 2017, respectively, posted with the same counterparties.
[2] CenterPoint Energy and CERC did not have significant Level 3 sales during any of the years ended December 31, 2018, 2017 or 2016.
[3] Mark-to-market value of Level 3 derivative assets acquired through the purchase of AEM were less than $1 million at the acquisition date.
[4] During 2016, CenterPoint Energy transferred its indexed debt securities from Level 2 to Level 3 to reflect changes in the significance of the unobservable inputs used in the valuation.
[5] During 2018, CenterPoint Energy transferred its indexed debt securities derivative from Level 3 to Level 2 to reflect changes in the significance of the unobservable inputs used in the valuation.
[6] Includes Securitization Bond debt.
[7] Level 1 natural gas derivatives include exchange-traded derivatives cleared by the CME, which deems that financial instruments cleared by the CME are settled daily in connection with posted cash payments. As a result of this exchange rule, CME-related derivatives are considered to have no fair value at the balance sheet date for financial reporting purposes, and are presented in Level 1 net of posted cash; however, the derivatives remain outstanding and subject to future commodity price fluctuations until they are settled in accordance with their contractual terms. Derivative transactions cleared on exchanges other than the CME (e.g., the Intercontinental Exchange or ICE) continue to be reported on a gross basis.
[8] Amounts are included in Prepaid and Other Current Assets and Other Assets in the Consolidated Balance Sheets.
[9] Natural gas derivatives include no material amounts related to physical forward transactions with Enable.