Fair Value Measurements (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Statement [Line Items]
Cash collateral posted with counterparties	$ 6,000,000				$ 6,000,000				$ 56,000,000
Corporate Equities	1,000,000				1,000,000				1,000,000
Investments, including money market funds	11,000,000	[1]			11,000,000	[1]			11,000,000	[1]
Natural gas derivatives, assets	51,000,000				51,000,000				107,000,000
Total Assets	63,000,000				63,000,000				119,000,000
Natural gas derivatives, liabilities	26,000,000				26,000,000				52,000,000
Total liabilities	26,000,000				26,000,000				52,000,000
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Beginning balance	3,000,000		5,000,000		6,000,000		3,000,000
Total unrealized gains (losses)	0	[2]	(1,000,000)	[2]	4,000,000	[2]	3,000,000	[2]
Total settlements	(2,000,000)	[2]	(1,000,000)	[2]	(8,000,000)	[2]	(3,000,000)	[2]
Transfers out of Level 3	0		0		(1,000,000)		0
Ending balance	1,000,000	[3]	3,000,000	[3]	1,000,000	[3]	3,000,000	[3]
Amount Of Total Gains For Period Included In Earnings	(1,000,000)		(1,000,000)		0		2,000,000
Carrying Amount [Member]
Statement [Line Items]
Long-term Debt	2,639,000,000				2,639,000,000				2,919,000,000
Fair Value [Member]
Statement [Line Items]
Long-term Debt	3,075,000,000				3,075,000,000				3,272,000,000
Fair Value, Inputs, Level 1 [Member]
Statement [Line Items]
Corporate Equities	1,000,000				1,000,000				1,000,000
Investments, including money market funds	11,000,000	[1]			11,000,000	[1]			11,000,000	[1]
Natural gas derivatives, assets	5,000,000				5,000,000				1,000,000
Total Assets	17,000,000				17,000,000				13,000,000
Natural gas derivatives, liabilities	8,000,000				8,000,000				19,000,000
Total liabilities	8,000,000				8,000,000				19,000,000
Fair Value, Inputs, Level 2 [Member]
Statement [Line Items]
Corporate Equities	0				0				0
Investments, including money market funds	0	[1]			0	[1]			0	[1]
Natural gas derivatives, assets	49,000,000				49,000,000				112,000,000
Total Assets	49,000,000				49,000,000				112,000,000
Natural gas derivatives, liabilities	28,000,000				28,000,000				101,000,000
Total liabilities	28,000,000				28,000,000				101,000,000
Fair Value, Inputs, Level 3 [Member]
Statement [Line Items]
Illiquid forward price low range (per mmbtu)	2.89				2.89
Illiquid forward price high range (per mmbtu)	4.90				4.90
Option volatilities low range	0.00%				0.00%
Option volatilities high range	67.00%				67.00%
Corporate Equities	0				0				0
Investments, including money market funds	0	[1]			0	[1]			0	[1]
Natural gas derivatives, assets	13,000,000				13,000,000				10,000,000
Total Assets	13,000,000				13,000,000				10,000,000
Natural gas derivatives, liabilities	12,000,000				12,000,000				4,000,000
Total liabilities	12,000,000				12,000,000				4,000,000
Netting Adjustments [Member]
Statement [Line Items]
Corporate Equities	0	[4]			0	[4]			0	[5]
Investments, including money market funds	0	[1],[4]			0	[1],[4]			0	[1],[5]
Natural gas derivatives, assets	(16,000,000)	[4]			(16,000,000)	[4]			(16,000,000)	[5]
Total Assets	(16,000,000)	[4]			(16,000,000)	[4]			(16,000,000)	[5]
Natural gas derivatives, liabilities	(22,000,000)	[4]			(22,000,000)	[4]			(72,000,000)	[5]
Total liabilities	$ (22,000,000)	[4]			$ (22,000,000)	[4]			$ (72,000,000)	[5]

[1] Excludes money market fund investments included in Cash and cash equivalents.

[2] CERC did not have Level 3 unrealized gain (losses) or settlements related to price stabilization activities of the Natural Gas Distribution business segment for either the three or nine months ended September 30, 2011 or 2012.

[3] During both the three and nine months ended September 30, 2011 and 2012, CERC did not have Level 3 purchases, sales or significant transfers into Level 3.

[4] Amounts represent the impact of legally enforceable master netting agreements that allow CERC to settle positive and negative positions and also include cash collateral of $6 million posted with the same counterparties.

[5] Amounts represent the impact of legally enforceable master netting agreements that allow CERC to settle positive and negative positions and also include cash collateral of $56 million posted with the same counterparties.