Derivative Instruments (Details)		12 Months Ended
		Dec. 31, 2017 USD ($) Bcf		Dec. 31, 2016 USD ($) Bcf		Dec. 31, 2015 USD ($)	Aug. 21, 2017 USD ($)
Derivative asset fair value		$ 218,000,000		$ 105,000,000
Derivative liability fair value		107,000,000		67,000,000
Collateral posted (held), net		19,000,000		(14,000,000)
Gain (loss) on derivative instruments not designated as hedging instruments		139,000,000		52,000,000		$ 29,000,000
Total fair value of derivative instruments that contain credit risk contingent features that are in a net liability position		2,000,000		1,000,000
Aggregate fair value of assets already posted as collateral		0		0
Credit risk contingent feature assets		2,000,000		0
Fair value of credit risk derivative assets		154,000,000	[1]	84,000,000
Gains (Losses) in Expense: Natural Gas [Member]
Change in unrealized gain (loss) on hedged item in fair value hedge		14,000,000		0		0
Gain (loss) on fair value hedges recognized in earnings	[2]	5,000,000		0		0
Weather Hedge Swap [Member] | Gains (Losses) in Revenue [Member]
Net gain (loss) on derivative		$ 0		$ 0		(4,000,000)
Natural gas derivative [Member]
Derivative gross volume notional amount (in bcf) | Bcf		1,795		1,035
Net amount presented in balance sheet	[3]	$ 130,000,000		$ 24,000,000
Derivative, Fair Value, Net	[4]	111,000,000		38,000,000
Collateral posted (held), net		19,000,000		(14,000,000)
Natural gas derivative [Member] | Current Assets [Member]
Derivative asset, gross amount recognized	[4]	165,000,000		81,000,000
Derivative assets gross amounts offset in balance sheet		(55,000,000)		(30,000,000)
Derivative Asset	[3]	110,000,000		51,000,000
Natural gas derivative [Member] | Other Noncurrent Assets [Member]
Derivative asset, gross amount recognized	[4]	53,000,000		24,000,000
Derivative assets gross amounts offset in balance sheet		(9,000,000)		(5,000,000)
Derivative Asset	[3]	44,000,000		19,000,000
Natural gas derivative [Member] | Current Liabilities [Member]
Derivative liability, gross amount recognized	[4]	(83,000,000)		(57,000,000)
Derivative liability gross amounts offset in balance sheet		63,000,000		16,000,000
Derivative Liability	[3]	(20,000,000)		(41,000,000)
Natural gas derivative [Member] | Other Noncurrent Liabilities [Member]
Derivative liability, gross amount recognized	[4]	(24,000,000)		(10,000,000)
Derivative liability gross amounts offset in balance sheet		20,000,000		5,000,000
Derivative Liability	[3]	(4,000,000)		(5,000,000)
Natural gas derivative [Member] | Gains (Losses) in Revenue [Member]
Gain (loss) on derivative instruments not designated as hedging instruments		211,000,000		(18,000,000)		134,000,000
Natural gas derivative [Member] | Gains (Losses) in Expense: Natural Gas [Member]
Change in unrealized gain (loss) on fair value hedging instruments		(9,000,000)		0		0
Gain (loss) on derivative instruments not designated as hedging instruments		(72,000,000)		70,000,000		$ (105,000,000)
Natural gas derivative [Member] | Designated as Hedging Instrument [Member] | Current Assets [Member]
Derivative asset fair value	[5],[6],[7]	0
Derivative liability fair value	[5],[6],[7]	0
Natural gas derivative [Member] | Designated as Hedging Instrument [Member] | Current Liabilities [Member]
Derivative asset fair value	[5],[6],[7]	13,000,000
Derivative liability fair value	[5],[6],[7]	1,000,000
Natural gas derivative [Member] | Not Designated as Hedging Instrument [Member] | Current Assets [Member]
Derivative asset fair value	[5]	114,000,000	[6],[7]	79,000,000	[8],[9]
Derivative liability fair value	[5]	4,000,000	[6],[7]	14,000,000	[8],[9]
Natural gas derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Noncurrent Assets [Member]
Derivative asset fair value	[5]	44,000,000	[6],[7]	24,000,000	[8],[9]
Derivative liability fair value	[5]	0	[6],[7]	5,000,000	[8],[9]
Natural gas derivative [Member] | Not Designated as Hedging Instrument [Member] | Current Liabilities [Member]
Derivative asset fair value	[5]	38,000,000	[6],[7]	2,000,000	[8],[9]
Derivative liability fair value	[5]	78,000,000	[6],[7]	43,000,000	[8],[9]
Natural gas derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivative asset fair value	[5]	9,000,000	[6],[7]	0	[8],[9]
Derivative liability fair value	[5]	$ 24,000,000	[6],[7]	$ 5,000,000	[8],[9]
Natural gas derivative [Member] | Long [Member]
Derivative gross volume notional amount (in bcf) | Bcf		224		59
Credit Risk Contract [Member]
Net amount presented in balance sheet		$ 154,000,000		$ 70,000,000
Collateral posted (held), net		0		(14,000,000)
Energy Marketers [Member]
Fair value of credit risk derivative assets		45,000,000		4,000,000
Financial Institutions [Member]
Fair value of credit risk derivative assets		0		33,000,000
Retail End Users [Member]
Fair value of credit risk derivative assets	[10]	109,000,000		47,000,000
External Credit Rating, Investment Grade [Member]
Fair value of credit risk derivative assets	[11]	23,000,000		36,000,000
External Credit Rating, Investment Grade [Member] | Energy Marketers [Member]
Fair value of credit risk derivative assets	[11]	6,000,000		1,000,000
External Credit Rating, Investment Grade [Member] | Financial Institutions [Member]
Fair value of credit risk derivative assets	[11]	0		33,000,000
External Credit Rating, Investment Grade [Member] | Retail End Users [Member]
Fair value of credit risk derivative assets	[10],[11]	17,000,000		$ 2,000,000
August [Member] | Treasury Lock [Member]
Derivative notional amount		150,000,000
Effective portion of realized losses		2,000,000
Senior Notes [Member]
Face amount of debt instrument							$ 300,000,000
Natural Gas Distribution [Member] | 2014To2015 [Member]
Weather hedge bilateral cap amount		16,000,000
Natural Gas Distribution [Member] | 2017To2018 [Member]
Weather hedge bilateral cap amount		$ 8,000,000

[1] The net of total non-trading natural gas derivative assets was $154 million and $70 million as of December 31, 2017 and 2016, respectively, as shown on CERC’s Consolidated Balance Sheets, and was comprised of the natural gas contracts derivatives assets separately shown above, impacted by collateral netting of $-0- and $14 million as of December 31, 2017 and 2016, respectively.

[2] Hedge ineffectiveness results from the basis ineffectiveness discussed above, and excludes the impact to natural gas expense from timing ineffectiveness. Timing ineffectiveness arises due to changes in the difference between the spot price and the futures price, as well as the difference between the timing of the settlement of the futures and the valuation of the underlying physical commodity. As the commodity contract nears the settlement date, spot-to-forward price differences should converge, which should reduce or eliminate the impact of this ineffectiveness on natural gas expense.

[3] The derivative assets and liabilities on the Consolidated Balance Sheets exclude accounts receivable or accounts payable that, should they exist, could be used as offsets to these balances in the event of a default.

[4] Gross amounts recognized include some derivative assets and liabilities that are not subject to master netting arrangements.

[5] Derivative Assets and Derivative Liabilities include no material amounts related to physical forward transactions with Enable.

[6] Natural gas contracts are presented on a net basis in the Consolidated Balance Sheets as they are subject to master netting arrangements. This netting applies to all undisputed amounts due or past due and causes derivative assets (liabilities) to be ultimately presented net in a liability (asset) account within the Consolidated Balance Sheets. The net of total non-trading natural gas derivative assets and liabilities was a $130 million asset as shown on CERC’s Consolidated Balance Sheets (and as detailed in the table below), and was comprised of the natural gas contracts derivative assets and liabilities separately shown above, impacted by collateral netting of $19 million.

[7] The fair value shown for natural gas contracts is comprised of derivative gross volumes totaling 1,795 Bcf or a net 224 Bcf long position. Certain natural gas contracts hedge basis risk only and lack a fixed price exposure.

[8] Natural gas contracts are presented on a net basis in the Consolidated Balance Sheets. Natural gas contracts are subject to master netting arrangements. This netting applies to all undisputed amounts due or past due and causes derivative assets (liabilities) to be ultimately presented net in a liability (asset) account within the Consolidated Balance Sheets. The net of total non-trading derivative assets and liabilities was a $24 million asset as shown on CERC’s Consolidated Balance Sheets (and as detailed in the table below), and was comprised of the natural gas contracts derivative assets and liabilities separately shown above, impacted by collateral netting of $14 million.

[9] The fair value shown for natural gas contracts is comprised of derivative gross volumes totaling 1,035 Bcf or a net 59 Bcf long position. Certain natural gas contracts hedge basis risk only and lack a fixed price exposure.

[10] End users are comprised primarily of customers who have contracted to fix the price of a portion of their physical gas requirements for future periods.

[11] “Investment grade” is primarily determined using publicly available credit ratings and considers credit support (including parent company guarantees) and collateral (including cash and standby letters of credit). For unrated counterparties, CERC determines a synthetic credit rating by performing financial statement analysis and considers contractual rights and restrictions and collateral.