Derivative Instruments Derivative Instruments (Details) $ in Millions		3 Months Ended			12 Months Ended
		Mar. 31, 2016 USD ($) Bcf		Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) Bcf
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments, Assets, Physical Forwards Purchased		$ 117			$ 140
Derivative Liability, Fair Value, Gross Liability		72			87
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		9		$ (1)
Derivative, Fair Value, Net [Abstract]
Total fair value of the derivative instruments that contain credit risk contingency features that are in a net liability position		2			3
Collateral Already Posted, Aggregate Fair Value		0			0
Credit Risk Contingent Feature assets		$ 2			$ 2
Energy Related Derivative [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative, Nonmonetary Notional Amount, Volume | Bcf		751			767
Gross Amounts Recognized (1)	[1]	$ 45			$ 53
Derivative, Fair Value, Offsets, Net		47			56
Derivative Assets (Liabilities), at Fair Value, Net	[2]	92			109
collateral Netting		47			56
Energy Related Derivative [Member] | Current Assets [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	[1]	85			100
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		(10)			(11)
Derivative Asset	[2]	75			89
Energy Related Derivative [Member] | Other Noncurrent Assets [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	[1]	32			40
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		(4)			(4)
Derivative Asset	[2]	28			36
Energy Related Derivative [Member] | Current Liabilities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[1]	(57)			(62)
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		49			51
Derivative Liability	[2]	(8)			(11)
Energy Related Derivative [Member] | Other Noncurrent Liabilities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[1]	(15)			(25)
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		12			20
Derivative Liability	[2]	(3)			(5)
Energy Related Derivative [Member] | Gains (Losses) in Revenue
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		20		(133)
Energy Related Derivative [Member] | Gains (Losses) in Expense: Natural Gas [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net		(11)		$ 132
Not Designated as Hedging Instrument [Member] | Energy Related Derivative [Member] | Current Assets [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments, Assets, Physical Forwards Purchased	[3]	75	[4],[5]		90	[6],[7]
Derivative Liability, Fair Value, Gross Liability	[3]	0	[4],[5]		2	[6],[7]
Not Designated as Hedging Instrument [Member] | Energy Related Derivative [Member] | Other Noncurrent Assets [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments, Assets, Physical Forwards Purchased	[3]	28	[4],[5]		36	[6],[7]
Derivative Liability, Fair Value, Gross Liability	[3]	0	[4],[5]		0	[6],[7]
Not Designated as Hedging Instrument [Member] | Energy Related Derivative [Member] | Current Liabilities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments, Assets, Physical Forwards Purchased	[3]	10	[4],[5]		10	[6],[7]
Derivative Liability, Fair Value, Gross Liability	[3]	57	[4],[5]		60	[6],[7]
Not Designated as Hedging Instrument [Member] | Energy Related Derivative [Member] | Other Noncurrent Liabilities [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments, Assets, Physical Forwards Purchased	[3]	4	[4],[5]		4	[6],[7]
Derivative Liability, Fair Value, Gross Liability	[3]	$ 15	[4],[5]		$ 25	[6],[7]
Long [Member] | Energy Related Derivative [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative, Nonmonetary Notional Amount, Volume | Bcf		139			112
Swap [Member] | Long [Member] | Energy Related Derivative [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative, Nonmonetary Notional Amount, Volume | Bcf		122			133

[1] Gross amounts recognized include some derivative assets and liabilities that are not subject to master netting arrangements.

[2] The derivative assets and liabilities on the Condensed 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] Derivative Assets and Derivative Liabilities include no material amounts related to physical forward transactions with Enable.

[4] Natural gas contracts are presented on a net basis in the Condensed Consolidated Balance Sheets as they 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 $92 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 $47 million.

[5] The fair value shown for natural gas contracts is comprised of derivative gross volumes totaling 751 Bcf or a net 139 Bcf long position. Of the net long position, basis swaps constitute 122 Bcf.

[6] Natural gas contracts are presented on a net basis in the Condensed Consolidated Balance Sheets as they 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 $109 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 $56 million.

[7] The fair value shown for natural gas contracts is comprised of derivative gross volumes totaling 767 Bcf or a net 112 Bcf long position. Of the net long position, basis swaps constitute 133 Bcf.