Retirement and Post-Retirement Employee Benefit Plans Defined Benefit Plans Disclosure (Details) (USD $)	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			0 Months Ended	12 Months Ended										12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Pension Plans, Defined Benefit [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member]	Sep. 30, 2012 Pension Plans, Defined Benefit [Member]	Sep. 30, 2014 Pension Plans, Defined Benefit [Member] Domestic Equity Securities [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Domestic Equity Securities [Member]	Sep. 30, 2014 Pension Plans, Defined Benefit [Member] International Equity Securities [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] International Equity Securities [Member]	Sep. 30, 2014 Pension Plans, Defined Benefit [Member] Fixed Income Securities [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Fixed Income Securities [Member]	Sep. 30, 2014 Pension Plans, Defined Benefit [Member] Company Stock [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Company Stock [Member]	Sep. 30, 2014 Pension Plans, Defined Benefit [Member] Other Investment Assets [Member]	Sep. 30, 2013 Pension Plans, Defined Benefit [Member] Other Investment Assets [Member]	Sep. 30, 2014 Pension Account Plan, Defined Benefit [Member]	Sep. 30, 2013 Pension Account Plan, Defined Benefit [Member]	Sep. 30, 2014 Mississippi Valley Gas Union Retirement Plan, Defined Benefit [Member]	Apr. 02, 2013 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Mar. 31, 2014 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Sep. 30, 2013 Supplemental Executive Retirement Plans, Defined Benefit [Member]	Sep. 30, 2014 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]	Oct. 02, 2013 Supplemental Executive Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 Supplemental Executive Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 1998 Supplemental Executive Retirement Plan, Defined Benefit [Member]	Sep. 30, 2014 2009 Supplemental Executive Retirement Plan, Defined Benefit [Member]	Sep. 30, 2014 Other Postretirement Benefit Plans, 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, 2014 Other Postretirement Benefit Plans, Defined Benefit [Member] Diversified Investment Funds [Member]	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Diversified Investment Funds [Member]	Sep. 30, 2014 Other Postretirement Benefit Plans, Defined Benefit [Member] Cash and Cash Equivalents [Member]	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Cash and Cash Equivalents [Member]	Sep. 30, 2014 Minimum [Member] Pension Plans, Defined Benefit [Member]	Sep. 30, 2014 Maximum [Member] Pension Plans, Defined Benefit [Member]	Sep. 30, 2014 Retirement Savings Plan [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Unrecognized transition obligation	$ 354,000	$ 628,000	$ 0	$ 0																		$ 0	$ 0						$ 354,000	$ 628,000
Unrecognized prior service cost	(8,095,000)	(6,052,000)	(1,927,000)	(91,000)																		0	0						(6,168,000)	(5,961,000)
Unrecognized actuarial loss	151,745,000	176,048,000	109,767,000	108,621,000																		34,447,000	31,466,000						7,531,000	35,961,000
Total unrecognized in net periodic cost	144,004,000	170,624,000	107,840,000	108,530,000																		34,447,000	31,466,000						1,717,000	30,628,000
Current Year Benefit Plan Contributions [Abstract]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year Minimum																													20,000,000
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year Maximum																													25,000,000
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						51.90%	46.50%	15.30%	16.10%	9.70%	14.90%	12.90%	12.60%	10.20%	9.90%																	99.70%	96.80%	0.30%	3.20%
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												1,700,000	1,600,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate			4.95%	4.04%	5.05%														4.21%	4.04%	4.21%	4.95%	4.04%	5.05%					4.95%	4.04%	5.05%
Rate of compensation increase			3.50%	3.50%	3.50%																	3.50%	3.50%	3.50%
Expected return on plan assets			7.25%	7.75%	7.75%																								4.60%	4.70%	5.00%
Initial Trend																													8.00%	8.00%	8.00%
Ultimate trend rate																													5.00%	5.00%	5.00%
Ultimate trend rate reached in																													2020	2019	2018
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate			4.43%	4.95%																	4.95%	4.43%	4.95%						4.43%	4.95%
Rate of compensation increase			3.50%	3.50%																	3.50%	3.50%	3.50%
Expected return on plan assets			7.25%	7.25%																									4.60%	4.60%
Initial trend rate																													7.50%	8.00%
Ultimate trend rate																													5.00%	5.00%
Ultimate trend reached in																													2020	2020
Interest crediting rate																												4.69%
Accumulated benefit obligation			466,182,000	446,133,000																	109,817,000	106,276,000	109,817,000
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year			455,799,000	480,031,000																117,080,000		117,080,000	130,186,000						312,148,000	308,315,000
Service cost			15,345,000	17,754,000	15,084,000																	3,607,000	3,039,000	2,108,000					16,784,000	18,800,000	16,353,000
Interest cost			22,330,000	19,334,000	21,568,000																	4,966,000	4,755,000	5,142,000					15,951,000	12,964,000	13,861,000
Plan participants' contributions																													4,435,000	3,815,000
Actuarial (gain) loss			26,611,000	(29,822,000)																		9,468,000	(6,451,000)						(18,963,000)	(13,801,000)
Benefits paid			(24,519,000)	(25,073,000)																		(5,085,000)	(4,375,000)		(16,800,000)				(13,580,000)	(14,458,000)
Plan amendments																1,972,000	0												1,657,000
Divestitures			0	(6,425,000)																									0	(3,487,000)
Settlements																						(16,817,000)	(10,074,000)
Benefit obligation at end of year			493,594,000	455,799,000	480,031,000																117,080,000	113,219,000	117,080,000	130,186,000					315,118,000	312,148,000	308,315,000
Funded Status Reconciliation [Abstract]
Funded status			(58,827,000)	(58,912,000)																	(117,080,000)	(113,219,000)	(117,080,000)						(180,297,000)	(205,735,000)
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			(58,827,000)	(58,912,000)																	(117,080,000)	(113,219,000)	(117,080,000)						(180,297,000)	(205,735,000)
Estimated company benefit payments
Estimated company benefit payments in year one																													22,177,000
Estimated company benefit payments in year two																													15,008,000
Estimated company benefit payments in year three																													17,551,000
Estimated company benefit payments in year four																													19,660,000
Estimated company benefit payments in year five																													21,302,000
Estimated company benefit payments in five fiscal years thereafter																													127,741,000
Estimated retiree benefit payments
Estimated retiree benefit payments in year one																													3,582,000
Estimated retiree benefit payments in year two																													4,615,000
Estimated retiree benefit payments in year three																													5,796,000
Estimated retiree benefit payments in year four																													7,148,000
Estimated retiree benefit payments in year five																													8,549,000
Estimated retiree benefit payments in five fiscal years thereafter																													63,524,000
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			33,592,000																			11,381,000							25,759,000
Expected future benefit payments in year two, total			32,811,000																			4,617,000							19,623,000
Expected future benefit payments in year three, total			33,131,000																			17,260,000							23,347,000
Expected future benefit payments in year four, total			33,501,000																			14,772,000							26,808,000
Expected future benefit payments in year five, total			34,846,000																			7,675,000							29,851,000
Expected future benefit payments in five fiscal years thereafter, total			182,998,000																			32,843,000							191,265,000
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																													4,533,000
Effect of One Percentage Point Decrease on Service and Interest Cost Components																													(3,700,000)
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation																													40,922,000
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation																													(34,169,000)
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 October 2, 2013, due to the retirement of one of our executives, we recognized a settlement loss of $4.5 million associated with our SEBP and made a $16.8 million benefit payment.	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 Settlements																			$ 3,200,000			$ (4,539,000)	$ (3,160,000)	$ 0	$ 4,500,000
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%