Fair Value Measurements (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Assets
Corporate equities	$ 1,000,000		$ 1,000,000
Investments, including money market funds	11,000,000		11,000,000
Natural gas derivatives, assets	42,000,000		107,000,000
Total assets	54,000,000		119,000,000
Liabilities
Natural gas derivatives, liabilities	16,000,000		52,000,000
Total liabilities	16,000,000		52,000,000
Cash collateral posted with counterparties	9,000,000		56,000,000
Carrying Amount [Member]
Liabilities
Long-term debt	2,641,000,000		2,919,000,000
Fair Value [Member]
Liabilities
Long-term debt	3,094,000,000		3,272,000,000
Derivative Financial Instruments Assets And Liabilities Net Member
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Beginning balance	6,000,000		3,000,000		(6,000,000)
Total Gains Or Losses:
Included in earnings	3,000,000		6,000,000		4,000,000
Included in regulatory assets	0		0		(1,000,000)
Total settlements:
Included in earnings	(6,000,000)		(3,000,000)		(2,000,000)
Included in regulatory assets	0		0		8,000,000
Total purchases	0		2,000,000		0
Transfers out of Level 3	(1,000,000)	[1]	(2,000,000)	[1]	0	[1]
Ending balance	2,000,000		6,000,000		3,000,000
Amount Of Total Gains For Period Included In Earnings	1,000,000		5,000,000		4,000,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets
Corporate equities	1,000,000		1,000,000
Investments, including money market funds	11,000,000		11,000,000
Natural gas derivatives, assets	1,000,000		1,000,000
Total assets	13,000,000		13,000,000
Liabilities
Natural gas derivatives, liabilities	5,000,000		19,000,000
Total liabilities	5,000,000		19,000,000
Significant Other Observable Inputs (Level 2) [Member]
Assets
Corporate equities	0		0
Investments, including money market funds	0		0
Natural gas derivatives, assets	40,000,000		112,000,000
Total assets	40,000,000		112,000,000
Liabilities
Natural gas derivatives, liabilities	21,000,000		101,000,000
Total liabilities	21,000,000		101,000,000
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Illiquid forward price low range	3.07
Illiquid forward price high range	4.79
Option volatilities low range	0.00%
Option volatilities high range	58.00%
Assets
Corporate equities	0		0
Investments, including money market funds	0		0
Natural gas derivatives, assets	7,000,000		10,000,000
Total assets	7,000,000		10,000,000
Liabilities
Natural gas derivatives, liabilities	5,000,000		4,000,000
Total liabilities	5,000,000		4,000,000
Netting Adjustments [Member]
Assets
Corporate equities	0	[2]	0	[3]
Investments, including money market funds	0	[2]	0	[3]
Natural gas derivatives, assets	(6,000,000)	[2]	(16,000,000)	[3]
Total assets	(6,000,000)	[2]	(16,000,000)	[3]
Liabilities
Natural gas derivatives, liabilities	(15,000,000)	[2]	(72,000,000)	[3]
Total liabilities	$ (15,000,000)	[2]	$ (72,000,000)	[3]

[1] During 2010, 2011 and 2012, CERC did not have material Level 3 sales or significant transfers into Level 3.

[2] 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 $9 million posted with the same counterparties.

[3] 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.