Retirement and Post-Retirement Employee Benefit Plans Defined Benefit Plans Disclosure (Details) (USD $)	12 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended					12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		0 Months Ended	6 Months Ended			12 Months Ended													12 Months Ended			0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2010 Pension Plans, Defined Benefit [Member]		Sep. 30, 2011 Pension Plans, Defined Benefit [Member]		Sep. 30, 2013 Pension Plans, Defined Benefit [Member]	Sep. 30, 2012 Pension Plans, Defined Benefit [Member]	Sep. 30, 2011 Pension Plans, Defined Benefit [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Domestic Equity Securities [Member]	Sep. 30, 2012 Pension Plans, Defined Benefit [Member] Domestic Equity Securities [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] International Equity Securities [Member]	Sep. 30, 2012 Pension Plans, Defined Benefit [Member] International Equity Securities [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Fixed Income Securities [Member]	Sep. 30, 2012 Pension Plans, Defined Benefit [Member] Fixed Income Securities [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Company Stock [Member]	Sep. 30, 2012 Pension Plans, Defined Benefit [Member] Company Stock [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Other Investment Assets [Member]	Sep. 30, 2012 Pension Plans, Defined Benefit [Member] Other Investment Assets [Member]	Sep. 30, 2013 Pension Account Plan, Defined Benefit [Member]	Sep. 30, 2013 Mississippi Valley Gas Union Retirement Plan, Defined Benefit [Member]	Apr. 02, 2013 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Sep. 30, 2013 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Mar. 31, 2013 Supplemental Executive Retirement Plans, Defined Benefit [Member]		Sep. 30, 2013 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Sep. 30, 2012 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Sep. 30, 2011 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Sep. 30, 2013 Supplemental Executive Benefit Plan, Defined Benefit [Member]	Sep. 30, 2013 1998 Supplemental Executive Retirement Plan, Defined Benefit [Member]	Sep. 30, 2013 2009 Supplemental Executive Retirement Plan, Defined Benefit [Member]	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member]	Sep. 30, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member]	Sep. 30, 2011 Other Postretirement Benefit Plans, Defined Benefit [Member]	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Diversified Investment Funds [Member]	Sep. 30, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member] Diversified Investment Funds [Member]	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Cash and Cash Equivalents [Member]	Sep. 30, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member] Cash and Cash Equivalents [Member]	Sep. 30, 2013 Minimum [Member] Pension Plans, Defined Benefit [Member]	Sep. 30, 2013 Maximum [Member] Pension Plans, Defined Benefit [Member]	Sep. 30, 2013 Retirement Savings Plan [Member]	Oct. 02, 2013 Subsequent Event [Member]	Oct. 02, 2013 Subsequent Event [Member] Supplemental Executive Benefit Plan, Defined Benefit [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Unrecognized transition obligation	$ 628,000	$ 1,709,000					$ 0	$ 0																		$ 0	$ 0					$ 628,000	$ 1,709,000
Unrecognized prior service cost	(6,052,000)	(7,643,000)					(91,000)	(232,000)																		0	0					(5,961,000)	(7,411,000)
Unrecognized actuarial loss	176,048,000	294,447,000					108,621,000	187,050,000																		31,466,000	43,995,000					35,961,000	63,402,000
Total unrecognized in net periodic cost	170,624,000	288,513,000					108,530,000	186,818,000																		31,466,000	43,995,000					30,628,000	57,700,000
Current Year Benefit Plan Contributions [Abstract]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year Minimum							15,000,000																									25,000,000
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year Maximum							25,000,000																									30,000,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum										35.00%		10.00%		10.00%		5.00%		5.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum										55.00%		20.00%		30.00%		15.00%		15.00%
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations										46.50%	42.60%	16.10%	13.90%	14.90%	18.60%	12.60%	12.00%	9.90%	12.90%																96.80%	97.00%	3.20%	3.00%
Number of shares of Atmos Energy common stock in Master Trust																1,169,700	1,169,700
Amount of dividend income from Atmos Energy common stock in Master Trust																1,600,000	1,600,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate			5.39%	[1]	5.68%	[1]	4.04%	5.05%														4.21%	4.21%	4.04%	[2]		5.05%	5.39%				4.04%	5.05%	5.39%
Rate of compensation increase							3.50%	3.50%	4.00%																	3.50%	3.50%	4.00%
Expected return on plan assets							7.75%	7.75%	8.25%																							4.70%	5.00%	5.00%
Initial Trend																																8.00%	8.00%	8.00%
Ultimate trend rate																																5.00%	5.00%	5.00%
Ultimate trend rate reached in																																2019	2018	2016
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate							4.95%	4.04%															4.95%			4.95%	4.04%					4.95%	4.04%
Rate of compensation increase							3.50%	3.50%															3.50%			3.50%	3.50%
Expected return on plan assets							7.25%	7.75%																								4.60%	4.70%
Initial trend rate																																8.00%	8.00%
Ultimate trend rate																																5.00%	5.00%
Ultimate trend reached in																																2020	2019
Interest crediting rate																															4.69%
Accumulated benefit obligation							446,133,000	468,440,000															109,817,000			109,817,000	121,815,000
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year							480,031,000	429,432,000																130,186,000		130,186,000	112,115,000					308,315,000	263,694,000
Service cost							17,754,000	15,084,000	14,384,000																	3,039,000	2,108,000	2,768,000				18,800,000	16,353,000	14,403,000
Interest cost							19,334,000	21,568,000	22,264,000																	4,755,000	5,142,000	5,825,000				12,964,000	13,861,000	12,813,000
Plan participants' contributions																																3,815,000	3,649,000
Actuarial (gain) loss							(29,822,000)	46,197,000																		(6,451,000)	15,459,000					(13,801,000)	28,815,000
Benefits paid							(25,073,000)	(24,553,000)																		(4,375,000)	(4,638,000)					(14,458,000)	(13,197,000)										(16,800,000)
Divestitures							(6,425,000)	(7,697,000)																								(3,487,000)	(4,860,000)
Settlements																										(10,074,000)	0
Benefit obligation at end of year					429,432,000		455,799,000	480,031,000	429,432,000														117,080,000			117,080,000	130,186,000	112,115,000				312,148,000	308,315,000	263,694,000
Funded Status Reconciliation [Abstract]
Funded status							(58,912,000)	(136,887,000)															(117,080,000)			(117,080,000)	(130,186,000)					(205,735,000)	(231,243,000)
Unrecognized prior service cost							0	0															0			0	0					0	0
Unrecognized transition obligation																																0	0
Unrecognized net loss							0	0															0			0	0					0	0
Net amount recognized							(58,912,000)	(136,887,000)															(117,080,000)			(117,080,000)	(130,186,000)					(205,735,000)	(231,243,000)
Estimated company benefit payments
Estimated company benefit payments in year one																																25,547,000
Estimated company benefit payments in year two																																16,628,000
Estimated company benefit payments in year three																																19,260,000
Estimated company benefit payments in year four																																21,216,000
Estimated company benefit payments in year five																																22,550,000
Estimated company benefit payments in five fiscal years thereafter																																116,617,000
Estimated retiree benefit payments
Estimated retiree benefit payments in year one																																3,899,000
Estimated retiree benefit payments in year two																																4,915,000
Estimated retiree benefit payments in year three																																6,049,000
Estimated retiree benefit payments in year four																																7,304,000
Estimated retiree benefit payments in year five																																8,677,000
Estimated retiree benefit payments in five fiscal years thereafter																																58,595,000
Disclosure Of Expected Gross Prescription Drug Subsidy Receipts Abstract
Prescription drug subsidy receipts in year one																																0
Prescription drug subsidy receipts in year two																																0
Prescription drug subsidy receipts in year three																																0
Prescription drug subsidy receipts in year four																																0
Prescription drug subsidy receipts in year five																																0
Prescription drug subsidy receipts in five fiscal years thereafter																																0
Estimated Future Benefits Payment, Total
Expected future benefit payments in year one, total							40,640,000																22,940,000			22,940,000						29,446,000
Expected future benefit payments in year two, total							36,230,000																6,363,000			6,363,000						21,543,000
Expected future benefit payments in year three, total							34,752,000																6,226,000			6,226,000						25,309,000
Expected future benefit payments in year four, total							33,612,000																6,440,000			6,440,000						28,520,000
Expected future benefit payments in year five, total							33,273,000																6,913,000			6,913,000						31,227,000
Expected future benefit payments in five fiscal years thereafter, total							156,367,000																34,260,000			34,260,000						175,212,000
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Effect of One Percentage Point Increase on Service and Interest Cost Components																																4,399,000
Effect of One Percentage Point Decrease on Service and Interest Cost Components																																(3,682,000)
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation																																36,680,000
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation																																(30,940,000)
Subsequent Events [Abstract]
Subsequent Event, Date																																										Oct. 02, 2013
Subsequent Event, Description																																										due to the retirement of one of our executives, we recognized a settlement loss of $4.5 million associated with our Supplemental Executive Benefits Plan. In association with the retirement, on October 2, 2013, we made a $16.8 million benefit payment from the SEBP.
Defined Benefit Plan, Employer Claims Paid																																80.00%
Defined Benefit Plan, Employee Paid Claims																																20.00%
Defined Benefit Plan, Percent of Covered Compensation																													75.00%	60.00%
Defined Benefit Plan, Vesting Period																				3 years	5 years										3 years										3 years
Defined Benefit Plan, Age Attained																															55 years
Defined Benefit Plan, Investment Performance Review Period																																							3 years	5 years
Defined Benefit Plan, Description of Settlements and Curtailments																										On April 1, 2013, due to the retirement of certain executives, we recognized a settlement loss of $3.2 million associated with the supplemental plans and revalued the net periodic pension cost for the remainder of fiscal 2013. The revaluation of the net periodic pension cost resulted in an increase in the discount rate, effective April 1, 2013, to 4.21 percent, which reduced our net periodic pension cost by approximately $0.1 million for the remainder of the fiscal year.
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments							0	0	40,000
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements																						$ 3,200,000				$ (3,160,000)	$ 0	$ 0															$ 4,500,000
Defined Benefit Plan, Net Rate of Return Goal							4.00%
Defined Benefit Plan, Funds Available for Investment in Common Stock and Convertible Securities																																75.00%
Defined Benefit Plan, Percent of Contributed Compensation																															10.00%
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	15 years
Defined Benefit Plan, Percent Limit on Future Contributions																																3.00%

[1] (1) The discount rate for the Pension Account Plan increased from 5.39% to 5.68% effective January 1, 2011 due to a curtailment gain recorded in fiscal 2011.

[2] (1)The discount rate for the supplemental plans increased from 4.04% to 4.21% effective April 1, 2013 due to a settlement loss recorded in fiscal 2013.