Retirement and Post-Retirement Employee Benefit Plans Defined Benefit Plans Disclosure (Details) - USD ($) $ in Thousands					6 Months Ended		12 Months Ended
	Nov. 04, 2015	Oct. 08, 2015	Oct. 02, 2013	Apr. 02, 2013	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Unrecognized transition obligation							$ 82	$ 354
Unrecognized prior service cost							(6,259)	(8,095)
Unrecognized actuarial loss							110,714	151,745
Total unrecognized in net periodic cost							$ 104,537	144,004
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided							15 years
Subsequent Event, AEH 401K Merger [Member]
Subsequent Event [Line Items]
Subsequent Event, Date	Nov. 04, 2015
Subsequent Event, Description	the Atmos Energy Corporation Board of Directors voted to approve the merger of the assets and liabilities of the AEH 401K Profit-Sharing Plan with the Retirement Savings Plan, effective January 1, 2016 to improve administrative efficiency and reduce total costs.
Subsequent Event, Mortality Tables Update [Member]
Subsequent Event [Line Items]
Subsequent Event, Date		Oct. 08, 2015
Subsequent Event, Description		the Society of Actuaries issued an additional report related to mortality tables and the mortality improvement scale.  As of September 30, 2015, we updated our assumed mortality rates to incorporate both new sets of mortality tables issued by the Society of Actuaries.
Pension Plans, Defined Benefit [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Unrecognized transition obligation							$ 0	0
Unrecognized prior service cost							(1,735)	(1,927)
Unrecognized actuarial loss							120,948	109,767
Total unrecognized in net periodic cost							$ 119,213	$ 107,840
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate							4.43%	4.95%	4.04%
Rate of compensation increase							3.50%	3.50%	3.50%
Expected return on plan assets							7.25%	7.25%	7.75%
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate							4.55%	4.43%
Rate of compensation increase							3.50%	3.50%
Expected return on plan assets							7.00%	7.25%
Accumulated benefit obligation							$ 485,921	$ 466,182
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year			$ 455,799				493,594	455,799
Service cost							16,231	15,345	$ 17,754
Interest cost							21,850	22,330	19,334
Actuarial (gain) loss							7,420	26,611
Benefits paid							(30,496)	(24,519)
Plan amendments							0	(1,972)
Benefit obligation at end of year					$ 455,799		508,599	493,594	$ 455,799
Funded Status of Plan Reconciliation
Funded status							(57,667)	(58,827)
Unrecognized prior service cost							0	0
Unrecognized net loss							0	0
Net amount recognized							(57,667)	$ (58,827)
Estimated Future Benefits Payment, Total
Expected future benefit payments in year one, total							30,909
Expected future benefit payments in year two, total							31,805
Expected future benefit payments in year three, total							33,086
Expected future benefit payments in year four, total							34,712
Expected future benefit payments in year five, total							36,128
Expected future benefit payments in five fiscal years thereafter, total							$ 201,857
Pension Plans, Defined Benefit [Member] | Minimum [Member]
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Defined Benefit Plan, Investment Performance Review Period							3 years
Pension Plans, Defined Benefit [Member] | Maximum [Member]
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Defined Benefit Plan, Investment Performance Review Period							5 years
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum							35.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum							55.00%
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations							41.30%	44.90%
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum							10.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum							20.00%
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations							14.90%	15.30%
Pension Plans, Defined Benefit [Member] | Fixed Income Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum							5.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum							30.00%
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations							11.00%	9.70%
Pension Plans, Defined Benefit [Member] | Company Stock [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum							0.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum							15.00%
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations							15.20%	12.90%
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							$ 1,800	$ 1,700
Pension Plans, Defined Benefit [Member] | Other Investment Assets [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum							0.00%
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum							20.00%
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations							17.60%	17.20%
Pension Account Plan, Defined Benefit [Member]
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Defined Benefit Plan, Vesting Period							3 years
Supplemental Employee Retirement Plan [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Unrecognized transition obligation							$ 0	$ 0
Unrecognized prior service cost							0	0
Unrecognized actuarial loss							36,915	34,447
Total unrecognized in net periodic cost							$ 36,915	$ 34,447
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate				4.21%	4.21%	4.04%	4.43%	4.95%	4.04%	[1]
Rate of compensation increase							3.50%	3.50%	3.50%
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate							4.55%	4.43%
Rate of compensation increase							3.50%	3.50%
Accumulated benefit obligation							$ 118,835	$ 106,276
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year			117,080				113,219	117,080
Service cost							3,971	3,607	$ 3,039
Interest cost							4,943	4,966	4,755
Actuarial (gain) loss							4,811	9,468
Benefits paid							(4,551)	(5,085)
Settlements							0	(16,817)
Benefit obligation at end of year					$ 117,080		122,393	113,219	117,080
Funded Status of Plan Reconciliation
Funded status							(122,393)	(113,219)
Unrecognized prior service cost							0	0
Unrecognized net loss							0	0
Net amount recognized							(122,393)	$ (113,219)
Estimated Future Benefits Payment, Total
Expected future benefit payments in year one, total							21,857
Expected future benefit payments in year two, total							13,901
Expected future benefit payments in year three, total							12,807
Expected future benefit payments in year four, total							6,808
Expected future benefit payments in year five, total							4,454
Expected future benefit payments in five fiscal years thereafter, total							47,047
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
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.
Settlements				$ 3,200			$ 0	$ 4,539	$ 3,160
Supplemental Executive Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefits paid			(16,800)
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Defined Benefit Plan, Percent of Covered Compensation							75.00%
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.
Settlements			4,500
1998 Supplemental Executive Retirement Plan, Defined Benefit [Member]
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Defined Benefit Plan, Percent of Covered Compensation							60.00%
2009 Supplemental Executive Retirement Plan, Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Interest crediting rate							4.69%
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Defined Benefit Plan, Vesting Period							3 years
Defined Benefit Plan, Age Attained							55 years
Defined Benefit Plan, Percent of Contributed Compensation							10.00%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Unrecognized transition obligation							$ 82	354
Unrecognized prior service cost							(4,524)	(6,168)
Unrecognized actuarial loss							(47,149)	7,531
Total unrecognized in net periodic cost							$ (51,591)	$ 1,717
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate							4.43%	4.95%	4.04%
Expected return on plan assets							4.60%	4.60%	4.70%
Initial Trend							7.50%	8.00%	8.00%
Ultimate trend rate							5.00%	5.00%	5.00%
Ultimate trend rate reached in					2019		2020	2020	2019
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate							4.55%	4.43%
Expected return on plan assets							4.45%	4.60%
Initial trend rate							7.50%	7.50%
Ultimate trend rate							5.00%	5.00%
Ultimate trend reached in							2021	2020
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year			$ 312,148				$ 315,118	$ 312,148
Service cost							15,583	16,784	$ 18,800
Interest cost							14,385	15,951	12,964
Plan participants' contributions							4,563	4,435
Actuarial (gain) loss							(69,962)	(18,963)
Benefits paid							(12,508)	(13,580)
Plan amendments							0	(1,657)
Benefit obligation at end of year					$ 312,148		267,179	315,118	$ 312,148
Funded Status of Plan Reconciliation
Funded status							(129,170)	(180,297)
Unrecognized prior service cost							0	0
Unrecognized transition obligation							0	0
Unrecognized net loss							0	0
Net amount recognized							(129,170)	$ (180,297)
Estimated company benefit payments
2006							15,498
2007							11,245
Estimated company benefit payments in year three							12,185
Estimated company benefit payments in year four							12,755
Estimated company benefit payments in year five							13,995
Estimated company benefit payments in five fiscal years thereafter							86,519
Estimated retiree benefit payments
Estimated retiree benefit payments in year one							2,382
Estimated retiree benefit payments in year two							2,677
Estimated retiree benefit payments in year three							2,980
Estimated retiree benefit payments in year four							3,324
Estimated retiree benefit payments in year five							3,766
Estimated retiree benefit payments in five fiscal years thereafter							26,363
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							17,880
Expected future benefit payments in year two, total							13,922
Expected future benefit payments in year three, total							15,165
Expected future benefit payments in year four, total							16,079
Expected future benefit payments in year five, total							17,761
Expected future benefit payments in five fiscal years thereafter, total							112,882
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							4,288
Effect of One Percentage Point Decrease on Service and Interest Cost Components							(3,426)
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation							39,572
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation							$ (32,440)
Defined Benefit Plan, Employer Claims Paid							80.00%
Defined Benefit Plan, Employee Paid Claims							20.00%
Defined Benefit Plan, Percent Limit on Future Contributions							3.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Minimum [Member]
Current Year Benefit Plan Contributions [Abstract]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year							$ 30,000
Other Postretirement Benefit Plans, Defined Benefit [Member] | Maximum [Member]
Current Year Benefit Plan Contributions [Abstract]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year							$ 40,000
Other Postretirement Benefit Plans, Defined Benefit [Member] | Diversified Investment Funds [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations							97.50%	99.70%
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Defined Benefit Plan, Funds Available for Investment in Common Stock and Convertible Securities							75.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations							2.50%	0.30%

[1] (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.