Derivative Instruments Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013 MMcf Years		Sep. 30, 2012		Dec. 31, 2012 MMcf
Derivatives, Fair Value [Line Items]
Weather Hedges Term					10
Derivative, Gain (Loss) on Derivative, Net	$ 0		$ 0		$ (6)		$ 6
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	9		3		21		(22)
Derivative Assets Fair Value	44				44				49
Derivative Liabilities Fair Value	(21)				(21)				(32)
Derivative, fair value, offsets, net	6				6				9
Net Amount Presented in the Consolidated Balance Sheets (2)	29				29				26
Derivative gross volumes on natural gas contracts (MMcf)					607,000				489,000
Net long position (MMcf)					24,000				101,000
Amount of net long position constituted by basis swaps (MMcf)					89,000				73,000
Derivative Instruments, Liabilities, Physical Forwards Purchased	2				2
Total fair value of the derivative instruments that contain credit risk contingency features that are in a net liability position	3				3				5
The aggregate fair value of assets already posted as collateral	1				1				1
Credit Risk Contingent Feature assets	3				3				5
Natural Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Recognized (1)	23	[1]			23	[1]			17	[1]
Derivative, fair value, offsets, net	6				6				9
Net Amount Presented in the Consolidated Balance Sheets (2)	29	[2]			29	[2]			26	[2]
Natural Gas Derivatives [Member] | Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	34	[1]			34	[1]			42	[1]
Derivative Asset, Fair Value, Gross Liability	(8)				(8)				(6)
Net Amount Presented in the Consolidated Balance Sheets (2)	26	[2]			26	[2]			36	[2]
Natural Gas Derivatives [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	10	[1]			10	[1]			7	[1]
Derivative Asset, Fair Value, Gross Liability	0				0				(1)
Net Amount Presented in the Consolidated Balance Sheets (2)	10	[2]			10	[2]			6	[2]
Natural Gas Derivatives [Member] | Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities Fair Value	(17)	[1]			(17)	[1]			(28)	[1]
Derivative Liability, Fair Value, Gross Asset	11				11				14
Net Amount Presented in the Consolidated Balance Sheets (2)	(6)	[2]			(6)	[2]			(14)	[2]
Natural Gas Derivatives [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities Fair Value	(4)	[1]			(4)	[1]			(4)	[1]
Derivative Liability, Fair Value, Gross Asset	3				3				2
Net Amount Presented in the Consolidated Balance Sheets (2)	(1)	[2]			(1)	[2]			(2)	[2]
Natural Gas Derivatives [Member] | Gains (Losses) in Revenue [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	11		(21)		24		22
Natural Gas Derivatives [Member] | Gains (Losses) in Expense: Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments Physical Forwards Purchased	0				(3)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(2)	[3]	24	[3]	(3)	[3],[4]	(44)	[3],[4]
Costs associated with price stabilization activities of the Natural Gas Distribution business segment included in Expenses					0		(38)
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member] | Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	26	[5],[6]			26	[5],[6]			37	[7],[8]
Derivative Liabilities Fair Value	0	[5],[6]			0	[5],[6]			(1)	[7],[8]
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	10	[5],[6]			10	[5],[6]			6	[7],[8]
Derivative Liabilities Fair Value	0	[5],[6]			0	[5],[6]			0	[7],[8]
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member] | Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	8	[5],[6],[9]			8	[5],[6],[9]			5	[7],[8]
Derivative Liabilities Fair Value	(17)	[5],[6],[9]			(17)	[5],[6],[9]			(27)	[7],[8]
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	0	[5],[6]			0	[5],[6]			1	[7],[8]
Derivative Liabilities Fair Value	$ (4)	[5],[6]			$ (4)	[5],[6]			$ (4)	[7],[8]

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

[2] 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.

[3] The Gains (Losses) in Expense: Natural Gas includes $-0- during the three months September 30, 2013 and $(3) million during the nine months ended September 30, 2013 related to physical forwards purchased from Enable.

[4] The Gains (Losses) in Expense: Natural Gas includes $(38) million and $-0- of costs during the nine months ended September 30, 2012 and 2013, respectively, associated with price stabilization activities of the Natural Gas Distribution business segment that will be ultimately recovered through purchased gas adjustments

[5] The fair value shown for natural gas contracts is comprised of derivative gross volumes totaling 607 Bcf or a net 24 Bcf long position. Of the net long position, basis swaps constitute 89 Bcf.

[6] Natural gas contracts are presented on a net basis in the Condensed 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 Condensed Consolidated Balance Sheets. The net of total non-trading derivative assets and liabilities was a $29 million asset as shown on CERC’s Condensed 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 offset by collateral netting of $6 million

[7] Natural gas contracts are presented on a net basis in the Condensed 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 Condensed Consolidated Balance Sheets. The net of total non-trading derivative assets and liabilities was a $26 million asset as shown on CERC’s Condensed 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 offset by collateral netting of $9 million

[8] The fair value shown for natural gas contracts is comprised of derivative gross volumes totaling 489 billion cubic feet (Bcf) or a net 101 Bcf long position. Of the net long position, basis swaps constitute 73 Bcf.

[9] The $17 million Derivative Current Liability includes $2 million related to physical forwards purchased from Enable.