Derivative Instruments Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2013 MMcf		Mar. 31, 2012		Dec. 31, 2012 MMcf
Derivatives, Fair Value [Line Items]
Weather Hedges Term	10
Gains (losses) recognized from weather hedges	$ (3)		$ 6
Gross Amounts Recognized	11	[1]			17	[1]
Gross Amounts Offset in the Consolidated Balance Sheets	0				9
Net Amount Presented in the Consolidated Balance Sheets	11	[2]			26	[2]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	2		(30)
Derivative Assets Fair Value	33				49
Derivative Liabilities Fair Value	22				32
Derivative gross volumes on natural gas contracts (MMcf)	492,000				489,000
Net long position (MMcf)	107,000				101,000
Amount of net long position constituted by basis swaps (MMcf)	57,000				73,000
Collateral Netting	1
Total fair value of the derivative instruments that contain credit risk contingency features that are in a net liability position	3				5
The aggregate fair value of assets already posted as collateral	1				1
Credit Risk Contingent Feature assets	2				5
Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Recognized	27	[1]			42	[1]
Gross Amounts Offset in the Consolidated Balance Sheets	(9)				(6)
Net Amount Presented in the Consolidated Balance Sheets	18	[2]			36	[2]
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Recognized	6	[1]			7	[1]
Gross Amounts Offset in the Consolidated Balance Sheets	(1)				(1)
Net Amount Presented in the Consolidated Balance Sheets	5	[2]			6	[2]
Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Recognized	(19)	[1]			(28)	[1]
Gross Amounts Offset in the Consolidated Balance Sheets	9				14
Net Amount Presented in the Consolidated Balance Sheets	(10)	[2]			(14)	[2]
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Recognized	(3)	[1]			(4)	[1]
Gross Amounts Offset in the Consolidated Balance Sheets	1				2
Net Amount Presented in the Consolidated Balance Sheets	(2)	[2]			(2)	[2]
Natural Gas Derivatives [Member] | Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	26	[3],[4]			37	[5],[6]
Derivative Liabilities Fair Value	8	[3],[4]			1	[5],[6]
Natural Gas Derivatives [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	5	[3],[4]			6	[5],[6]
Derivative Liabilities Fair Value	1	[3],[4]			0	[5],[6]
Natural Gas Derivatives [Member] | Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	1	[3],[4]			5	[5],[6]
Derivative Liabilities Fair Value	11	[3],[4]			27	[5],[6]
Natural Gas Derivatives [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets Fair Value	1	[3],[4]			1	[5],[6]
Derivative Liabilities Fair Value	2	[3],[4]			4	[5],[6]
Natural Gas Derivatives [Member] | Gains (Losses) in Revenue [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(14)		51
Natural Gas Derivatives [Member] | Gains (Losses) in Expense: Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	16	[7]	(81)	[7]
Costs associated with price stabilization activities of the Natural Gas Distribution business segment included in Expenses	$ 0		$ (38)

[1] Gross Amounts Recognized contain 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 fair value shown for natural gas contracts is comprised of derivative gross volumes totaling 492 Bcf or a net 107 Bcf long position. Of the net long position, basis swaps constitute 57 Bcf.

[4] 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 an $11 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 less than $1 million

[5] 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.

[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 $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:

[7] The Gains (Losses) in Expense: Natural Gas includes $(38) million and $-0- of costs in 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