Fair Value Measurements (Details)		3 Months Ended			9 Months Ended
		Sep. 30, 2017 USD ($)		Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)		Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value amount of Level 1 to Level 2 assets transferred		$ 0			$ 0
Fair value amount of Level 2 to Level 1 assets transferred		0			0
Fair value amount of Level 1 to Level 2 liabilities transferred		0			0
Fair value amount of Level 2 to Level 1 liabilities transferred		0			0
Assets
Total derivative assets, netting adjustment		(44,000,000)	[1]		(44,000,000)	[1]		$ (35,000,000)	[2]
Total assets		199,000,000			199,000,000			83,000,000
Derivative fair value offsets, net		13,000,000			13,000,000			(14,000,000)
Liabilities
Total derivative liabilities, netting adjustment		(57,000,000)	[1]		(57,000,000)	[1]		(21,000,000)	[2]
Total liabilities		27,000,000			27,000,000			46,000,000
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance		28,000,000		$ 16,000,000	13,000,000		$ 12,000,000
Purchases (1)	[3]	0		0	0		12,000,000
Total gains		(2,000,000)		9,000,000	21,000,000		13,000,000
Total settlements		(1,000,000)		(8,000,000)	(5,000,000)		(24,000,000)
Transfers into Level 3		7,000,000		0	9,000,000		5,000,000
Transfers out of Level 3		(6,000,000)		0	(12,000,000)		(1,000,000)
Ending balance (2)	[4]	26,000,000		17,000,000	26,000,000		17,000,000
The amount of total gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at the reporting date		0		$ 6,000,000	17,000,000		$ 14,000,000
Carrying Amount [Member]
Estimated Fair Value of FInancial Instruments
Long-term debt		2,636,000,000			2,636,000,000			2,375,000,000
Fair Value [Member]
Estimated Fair Value of FInancial Instruments
Long-term debt		2,854,000,000			$ 2,854,000,000			2,551,000,000
Atmos Energy Marketing [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Mark-to-market value of Level 3 derivative assets acquired					less than $1 million
Energy Related Derivative [Member]
Assets
Derivative fair value offsets, net		13,000,000			$ 13,000,000			(14,000,000)
Fair Value, Inputs, Level 1 [Member]
Assets
Total assets		82,000,000			82,000,000			24,000,000
Liabilities
Total liabilities		3,000,000			3,000,000			4,000,000
Fair Value, Inputs, Level 2 [Member]
Assets
Total assets		128,000,000			128,000,000			74,000,000
Liabilities
Total liabilities		74,000,000			74,000,000			56,000,000
Fair Value, Inputs, Level 3 [Member]
Assets
Total assets		33,000,000			33,000,000			20,000,000
Liabilities
Total liabilities		7,000,000			$ 7,000,000			7,000,000
Fair Value, Inputs, Level 3 [Member] | Options Held [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value options volatility rate					0.00%
Fair Value, Inputs, Level 3 [Member] | Options Held [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value options volatility rate					87.00%
Fair Value, Inputs, Level 3 [Member] | Forward Contracts [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value inputs (price per MMBtu)					1.08
Fair Value, Inputs, Level 3 [Member] | Forward Contracts [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value inputs (price per MMBtu)					5.83
Fair Value, Measurements, Recurring [Member]
Assets
Corporate equities		3,000,000			$ 3,000,000			3,000,000
Investments, including money market funds (2)	[5]	11,000,000			11,000,000			10,000,000
Hedged portion of natural gas inventory		65,000,000			65,000,000
Fair Value, Measurements, Recurring [Member] | Energy Related Derivative [Member]
Assets
Derivative asset, netting adjustment	[6]	(44,000,000)	[1]		(44,000,000)	[1]		(35,000,000)	[2]
Derivative assets	[6]	120,000,000			120,000,000			70,000,000
Liabilities
Derivative liabilities, netting adjustment	[6]	(57,000,000)	[1]		(57,000,000)	[1]		(21,000,000)	[2]
Derivative liabilities	[6]	27,000,000			27,000,000			46,000,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Corporate equities		3,000,000			3,000,000			3,000,000
Investments, including money market funds (2)	[5]	11,000,000			11,000,000			10,000,000
Hedged portion of natural gas inventory		65,000,000			65,000,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Energy Related Derivative [Member]
Assets
Derivative asset	[6]	3,000,000			3,000,000			11,000,000
Liabilities
Derivative liability	[6]	3,000,000			3,000,000			4,000,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Corporate equities		0			0			0
Investments, including money market funds (2)	[5]	0			0			0
Hedged portion of natural gas inventory		0			0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Energy Related Derivative [Member]
Assets
Derivative asset	[6]	128,000,000			128,000,000			74,000,000
Liabilities
Derivative liability	[6]	74,000,000			74,000,000			56,000,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Corporate equities		0			0			0
Investments, including money market funds (2)	[5]	0			0			0
Hedged portion of natural gas inventory		0			0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Energy Related Derivative [Member]
Assets
Derivative asset	[6]	33,000,000			33,000,000			20,000,000
Liabilities
Derivative liability	[6]	$ 7,000,000			$ 7,000,000			$ 7,000,000

[1] 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 $13 million posted with the same counterparties.

[2] 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 $14 million held by CES from the same counterparties.

[3] Mark-to-market value of Level 3 derivative assets acquired through the purchase of AEM was less than $1 million at the acquisition date.

[4] CERC did not have significant Level 3 sales during either of the three or nine months ended September 30, 2017 or 2016.

[5] Amounts are included in Other Assets in the Condensed Consolidated Balance Sheets.

[6] Natural gas derivatives include no material amounts related to physical forward transactions with Enable.