Fair Value Measurements (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Statement [Line Items]
Cash collateral posted with counterparties	$ 4,000,000				$ 4,000,000				$ 9,000,000
Corporate Equities	2,000,000				2,000,000				1,000,000
Investments, including money market funds	10,000,000				10,000,000				11,000,000
Natural gas derivatives, assets	34,000,000				34,000,000				42,000,000
Total Assets	46,000,000				46,000,000				54,000,000
Natural gas derivatives, liabilities	13,000,000				13,000,000				16,000,000
Total liabilities	13,000,000				13,000,000				16,000,000
Derivative Instruments, Liabilities, Physical Forwards Purchased	3,000,000				3,000,000
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Beginning balance	3,000,000		3,000,000		2,000,000		6,000,000
Total gains	1,000,000	[1]	2,000,000	[1]	3,000,000	[1]	4,000,000	[1]
Total settlements	0	[1]	(2,000,000)	[1]	(1,000,000)	[1]	(6,000,000)	[1]
Transfers out of Level 3	0		0		0		(1,000,000)
Ending balance	4,000,000	[2]	3,000,000	[2]	4,000,000	[2]	3,000,000	[2]
Amount Of Total Gains For Period Included In Earnings	2,000,000		1,000,000		3,000,000		2,000,000
Carrying Amount [Member]
Statement [Line Items]
Notes Receivable, Fair Value Disclosure	363,000,000				363,000,000				0
Long-term Debt	2,118,000,000				2,118,000,000				2,641,000,000
Fair Value [Member]
Statement [Line Items]
Notes Receivable, Fair Value Disclosure	360,000,000				360,000,000				0
Long-term Debt	2,374,000,000				2,374,000,000				3,094,000,000
Fair Value, Inputs, Level 1 [Member]
Statement [Line Items]
Corporate Equities	2,000,000				2,000,000				1,000,000
Investments, including money market funds	10,000,000				10,000,000				11,000,000
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	6,000,000				6,000,000				5,000,000
Total liabilities	6,000,000				6,000,000				5,000,000
Fair Value, Inputs, Level 2 [Member]
Statement [Line Items]
Corporate Equities	0				0				0
Investments, including money market funds	0				0				0
Natural gas derivatives, assets	33,000,000				33,000,000				40,000,000
Total Assets	33,000,000				33,000,000				40,000,000
Natural gas derivatives, liabilities	19,000,000	[3]			19,000,000	[3]			21,000,000
Total liabilities	19,000,000				19,000,000				21,000,000
Fair Value, Inputs, Level 3 [Member]
Statement [Line Items]
Illiquid forward price low range	2.89				2.89
Illiquid forward price high range	4.57				4.57
Option volatilities low range	0.00%				0.00%
Option volatilities high range	90.00%				90.00%
Corporate Equities	0				0				0
Investments, including money market funds	0				0				0
Natural gas derivatives, assets	7,000,000				7,000,000				7,000,000
Total Assets	7,000,000				7,000,000				7,000,000
Natural gas derivatives, liabilities	3,000,000				3,000,000				5,000,000
Total liabilities	3,000,000				3,000,000				5,000,000
Netting Adjustments [Member]
Statement [Line Items]
Corporate Equities	0	[4]			0	[4]			0	[5]
Investments, including money market funds	0	[4]			0	[4]			0	[5]
Natural gas derivatives, assets	(11,000,000)	[4]			(11,000,000)	[4]			(6,000,000)	[5]
Total Assets	(11,000,000)	[4]			(11,000,000)	[4]			(6,000,000)	[5]
Natural gas derivatives, liabilities	(15,000,000)	[4]			(15,000,000)	[4]			(15,000,000)	[5]
Total liabilities	$ (15,000,000)	[4]			$ (15,000,000)	[4]			$ (15,000,000)	[5]

[1] CERC did not have Level 3 unrealized gains (losses) or settlements related to price stabilization activities of the Natural Gas Distribution business segment during either the three or six months ended June 30, 2012 or 2013.

[2] CERC did not have significant Level 3 purchases, sales or transfers into Level 3 during either the three or six months ended June 30, 2012 or 2013.

[3] The (Level 2) Natural gas derivative liability of $19 million includes $3 million related to physical forwards purchased from Midstream Partnership.

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

[5] Amounts represent the impact of legally enforceable master netting arrangements that allow CERC to settle positive and negative positions and also include cash collateral of $9 million posted with the same counterparties.