Fair Value Measurements (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011		Dec. 31, 2011
Statement [Line Items]
Illiquid forward price low range (per mmbtu)	$ 2.52				$ 2.52
Illiquid forward price high range (per mmbtu)	4.64				4.64
Option volatilities low range	0.00%				0.00%
Option volatilities high range	90.00%				90.00%
Cash collateral posted with counterparties	20,000,000				20,000,000				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	89,000,000				89,000,000				107,000,000
Total Assets	101,000,000				101,000,000				119,000,000
Natural gas derivatives, liabilities	41,000,000				41,000,000				52,000,000
Total liabilities	41,000,000				41,000,000				52,000,000
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Beginning balance	3,000,000		6,000,000		6,000,000		3,000,000
Total unrealized gains	2,000,000	[2]	1,000,000	[2]	4,000,000	[2]	4,000,000	[2]
Total settlements	(2,000,000)	[2]	(2,000,000)	[2]	(6,000,000)	[2]	(2,000,000)	[2]
Transfers out of Level 3	0		0		(1,000,000)		0
Ending balance	3,000,000	[3]	5,000,000	[3]	3,000,000	[3]	5,000,000	[3]
Amount Of Total Gains For Period Included In Earnings	1,000,000		1,000,000		2,000,000		3,000,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Statement [Line Items]
Long-term Debt, Fair Value	2,981,000,000				2,981,000,000				3,272,000,000
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Statement [Line Items]
Long-term Debt, Fair Value	2,637,000,000				2,637,000,000				2,919,000,000
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	3,000,000				3,000,000				1,000,000
Total Assets	15,000,000				15,000,000				13,000,000
Natural gas derivatives, liabilities	11,000,000				11,000,000				19,000,000
Total liabilities	11,000,000				11,000,000				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	71,000,000				71,000,000				112,000,000
Total Assets	71,000,000				71,000,000				112,000,000
Natural gas derivatives, liabilities	38,000,000				38,000,000				101,000,000
Total liabilities	38,000,000				38,000,000				101,000,000
Fair Value, Inputs, Level 3 [Member]
Statement [Line Items]
Corporate Equities	0				0				0
Investments, including money market funds	0	[1]			0	[1]			0	[1]
Natural gas derivatives, assets	29,000,000				29,000,000				10,000,000
Total Assets	29,000,000				29,000,000				10,000,000
Natural gas derivatives, liabilities	26,000,000				26,000,000				4,000,000
Total liabilities	26,000,000				26,000,000				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	(14,000,000)	[4]			(14,000,000)	[4]			(16,000,000)	[5]
Total Assets	(14,000,000)	[4]			(14,000,000)	[4]			(16,000,000)	[5]
Natural gas derivatives, liabilities	(34,000,000)	[4]			(34,000,000)	[4]			(72,000,000)	[5]
Total liabilities	$ (34,000,000)	[4]			$ (34,000,000)	[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 six months ended June 30, 2011 or 2012.

[3] During both the three and six months ended June 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 $20 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.