Retirement and Post-Retirement Employee Benefit Plans Defined Benefit Plans Disclosure (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2015
Defined Benefit Plan [Abstract]
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	15 years
Available-for-sale Securities, Supplemental Executive Benefit Plans	$ 42,900	$ 41,300
Pension and Other Postretirement Plans Costs [Member] | Unrecognized Prior Service Credit [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	31	(4,389)
Pension and Other Postretirement Plans Costs [Member] | Unrecognized Actuarial (Gain) Loss [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	17,362	124,288
Pension and Other Postretirement Plans Costs [Member] | Total Unrecognized in Net Periodic Pension Cost [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	$ 17,393	$ 119,899
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	3.73%	4.55%		4.43%
Rate of compensation increase	3.50%	3.50%		3.50%
Expected return on plan assets	7.00%	7.00%		7.25%
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	3.89%	3.73%
Rate of compensation increase	3.50%	3.50%
Expected return on plan assets	6.75%	7.00%
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Accumulated benefit obligation	$ 505,355	$ 516,924
Benefit obligation at beginning of year	545,480	508,599
Service cost	18,109	16,419		$ 16,231
Interest cost	20,443	23,193		21,850
Actuarial (gain) loss	(16,347)	41,847
Benefits paid	(34,230)	(44,578)	[1]
Benefit obligation at end of year	533,455	545,480		$ 508,599
Funded Status of Plan Reconciliation
Funded status	(25,211)	(71,530)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	(25,211)	$ (71,530)
Estimated Future Benefits Payment, Total
FY2018	32,173
FY2019	32,903
FY2020	34,314
FY2021	36,487
FY2022	37,857
2023-2027	$ 204,690
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Vesting Period	3 years
Defined Benefit Plan, One-Time Voluntary Pension Buyout	$ 12,800
Pension Plans, Defined Benefit [Member] | Minimum [Member]
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Investment Performance Review Period	3 years
Pension Plans, Defined Benefit [Member] | Maximum [Member]
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Investment Performance Review Period		5 years
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations	43.90%	40.50%
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	35.00%
Pension Plans, Defined Benefit [Member] | Domestic Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	55.00%
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations	17.20%	15.50%
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%
Pension Plans, Defined Benefit [Member] | International Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	20.00%
Pension Plans, Defined Benefit [Member] | Fixed Income Securities [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations	10.60%	11.20%
Pension Plans, Defined Benefit [Member] | Fixed Income Securities [Member] | Minimum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%
Pension Plans, Defined Benefit [Member] | Fixed Income Securities [Member] | Maximum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	30.00%
Pension Plans, Defined Benefit [Member] | Company Stock [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations	11.80%	15.10%
Number of shares of Atmos Energy common stock in Master Trust	716,700	956,700
Amount of dividend income from Atmos Energy common stock in Master Trust	$ 1,700	$ 1,800
Pension Plans, Defined Benefit [Member] | Company Stock [Member] | Minimum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Pension Plans, Defined Benefit [Member] | Company Stock [Member] | Maximum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	15.00%
Pension Plans, Defined Benefit [Member] | Other Investment Assets [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Defined Benefit Plan, Actual Plan Asset Allocations	16.50%	17.70%
Pension Plans, Defined Benefit [Member] | Other Investment Assets [Member] | Minimum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Pension Plans, Defined Benefit [Member] | Other Investment Assets [Member] | Maximum [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	20.00%
Pension Plans, Defined Benefit [Member] | Pension Costs [Member] | Unrecognized Prior Service Credit [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	$ (1,278)	$ (1,509)
Pension Plans, Defined Benefit [Member] | Pension Costs [Member] | Unrecognized Actuarial (Gain) Loss [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	62,388	127,028
Pension Plans, Defined Benefit [Member] | Pension Costs [Member] | Total Unrecognized in Net Periodic Pension Cost [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	61,110	125,519
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement	$ 8,554	$ 0
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	3.73%	4.55%		4.43%
Rate of compensation increase	3.50%	3.50%		3.50%
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	3.89%	3.73%
Rate of compensation increase	3.50%	3.50%
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Accumulated benefit obligation	$ 130,070	$ 137,616
Benefit obligation at beginning of year	142,574	122,393
Service cost	2,756	2,371		$ 3,971
Interest cost	4,744	5,185		4,943
Actuarial (gain) loss	(2,452)	17,229
Benefits paid	(4,588)	(4,604)
Settlements	(8,554)	0
Benefit obligation at end of year	134,480	142,574		122,393
Funded Status of Plan Reconciliation
Funded status	(134,480)	(142,574)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	(134,480)	(142,574)
Estimated Future Benefits Payment, Total
FY2018	18,411
FY2019	23,000
FY2020	4,701
FY2021	4,609
FY2022	17,520
2023-2027	$ 48,415
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Changes in Benefit Obligation and Plan Assets, Description	Due to the departure of certain executives in February 2017, we recognized a settlement loss of $2.7 million associated with our SEBP and made an $8.6 million benefit payment during the fourth quarter of fiscal 2017.
Settlements	$ 2,685	0		$ 0
Supplemental Employee Retirement Plan [Member] | Supplemental Executive Benefit Plan Costs [Member] | Unrecognized Prior Service Credit [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	0	0
Supplemental Employee Retirement Plan [Member] | Supplemental Executive Benefit Plan Costs [Member] | Unrecognized Actuarial (Gain) Loss [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	42,170	51,558
Supplemental Employee Retirement Plan [Member] | Supplemental Executive Benefit Plan Costs [Member] | Total Unrecognized in Net Periodic Pension Cost [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	$ 42,170	$ 51,558
Supplemental Employee Retirement Plan [Member] | Supplemental Executive Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Percent of Covered Compensation	75.00%
Supplemental Employee Retirement Plan [Member] | 1998 Supplemental Executive Retirement Plan, Defined Benefit [Member]
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Percent of Covered Compensation	60.00%
Supplemental Employee Retirement Plan [Member] | 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%
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Vesting Period	3 years
Defined Benefit Plan, Age Attained	55 years
Defined Benefit Plan, Percent of Contributed Compensation	10.00%
Defined Benefit Plan, Percent of Contributed Compensation for Management Committee	25.00%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	3.73%	4.55%		4.43%
Expected return on plan assets	4.45%	4.45%		4.60%
Initial Trend	7.50%	7.50%		7.50%
Ultimate trend rate	5.00%	5.00%		5.00%
Ultimate trend rate reached in	2022	2021		2020
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	3.89%	3.73%
Expected return on plan assets	4.29%	4.45%
Initial trend rate	7.00%	7.50%
Ultimate trend rate	5.00%	5.00%
Ultimate trend reached in	2022	2022
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year	$ 279,222	$ 267,179
Service cost	12,436	10,823		$ 15,583
Interest cost	10,679	12,424		14,385
Plan participants' contributions	4,936	4,289
Actuarial (gain) loss	(21,750)	(1,052)
Benefits paid	(13,970)	(14,441)
Plan amendments	2,545	0
Benefit obligation at end of year	274,098	279,222		$ 267,179
Funded Status of Plan Reconciliation
Funded status	(89,308)	(120,245)
Unrecognized prior service cost	0	0
Unrecognized transition obligation	0	0
Unrecognized net loss	0	0
Net amount recognized	(89,308)	$ (120,245)
Estimated company benefit payments
FY2018	15,387
FY2019	12,140
FY2020	12,658
FY2021	13,571
FY2022	14,523
2023-2027	85,118
Estimated retiree benefit payments
FY2018	3,392
FY2019	3,751
FY2020	4,171
FY2021	4,704
FY2022	5,282
2023-2027	35,165
Disclosure Of Expected Gross Prescription Drug Subsidy Receipts Abstract
FY2018	0
FY2019	0
FY2020	0
FY2021	0
FY2022	0
2023-2027	0
Estimated Future Benefits Payment, Total
FY2018	18,779
FY2019	15,891
FY2020	16,829
FY2021	18,275
FY2022	19,805
2023-2027	120,283
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	4,526
Effect of One Percentage Point Decrease on Service and Interest Cost Components	(3,584)
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	41,259
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	$ (33,863)
Defined Benefit Plan [Abstract]
Defined Benefit Plan, Employer Claims Paid	80.00%
Defined Benefit Plan, Employee Paid Claims	20.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Minimum [Member]
Current Year Benefit Plan Contributions [Abstract]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 10,000
Other Postretirement Benefit Plans, Defined Benefit [Member] | Maximum [Member]
Current Year Benefit Plan Contributions [Abstract]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 20,000
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%	97.20%
Defined Benefit Plan [Abstract]
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%	2.80%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Postretirement Benefit Costs [Member] | Unrecognized Prior Service Credit [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	$ 1,309	$ (2,880)
Other Postretirement Benefit Plans, Defined Benefit [Member] | Postretirement Benefit Costs [Member] | Unrecognized Actuarial (Gain) Loss [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	(87,196)	(54,298)
Other Postretirement Benefit Plans, Defined Benefit [Member] | Postretirement Benefit Costs [Member] | Total Unrecognized in Net Periodic Pension Cost [Member]
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]
Net Regulatory Assets	$ (85,887)	$ (57,178)

[1] Includes $12.8 million of one-time payments to eligible deferred vested participants who elected to receive a lump-sum payout of their pension benefits during fiscal 2016.