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Retirement and Post-Retirement Employee Benefit Plans Defined Benefit Plans Disclosure (Details) - USD ($)
$ in Thousands
 12 Months Ended
Jan. 01, 2017
Oct. 25, 2016
Oct. 20, 2016
Oct. 05, 2016
Oct. 02, 2013
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [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           $ 41,300 $ 41,700  
Pension and Other Postretirement Plans Costs [Member] | Unrecognized Transition Obligation [Member]                
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]                
Net Regulatory Assets             82  
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           (4,389) (6,259)  
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           124,288 110,714  
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           $ 119,899 $ 104,537  
Subsequent Event [Member]                
Subsequent Event [Line Items]                
Subsequent Event, Date Jan. 01, 2017 Oct. 25, 2016 Oct. 20, 2016 Oct. 05, 2016        
Subsequent Event, Description  
On October 25, 2016, the uncommitted $25 million bilateral credit facility was renewed through July 31, 2017. These facilities are used primarily to issue letters of credit.
the Society of Actuaries released its annually-updated mortality improvement scale for pension plans incorporating new assumptions surrounding life expectancies in the United States.  As of September 30, 2016, we updated our assumed mortality rates to incorporate the updated mortality table.
we amended our existing $1.25 billion unsecured credit facility (the five-year unsecured credit facility) which increased the committed loan to $1.5 billion and extended the facility through September 25, 2021.
       
Pension Plans, Defined Benefit [Member]                
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract                
Discount rate           4.55% 4.43% 4.95%
Rate of compensation increase           3.50% 3.50% 3.50%
Expected return on plan assets           7.00% 7.25% 7.25%
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract                
Discount rate           3.73% 4.55%  
Rate of compensation increase           3.50% 3.50%  
Expected return on plan assets           7.00% 7.00%  
Defined Benefit Plan Change In Benefit Obligation Roll Forward                
Accumulated benefit obligation           $ 516,924 $ 485,921  
Benefit obligation at beginning of year           508,599 493,594  
Service cost           16,419 16,231 $ 15,345
Interest cost           23,193 21,850 22,330
Actuarial (gain) loss           41,847 7,420  
Benefits paid           (44,578) [1] (30,496)  
Benefit obligation at end of year           545,480 508,599 $ 493,594
Funded Status of Plan Reconciliation                
Funded status           (71,530) (57,667)  
Unrecognized prior service cost           0 0  
Unrecognized net loss           0 0  
Net amount recognized           (71,530) $ (57,667)  
Estimated Future Benefits Payment, Total                
FY2017           31,306    
FY2018           32,047    
FY2019           33,674    
FY2020           35,232    
FY2021           37,279    
2022-2026           202,442    
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
Defined Benefit Plan, One-Time Voluntary Pension Buyout           $ 12,800    
Pension Plans, Defined Benefit [Member] | Minimum [Member]                
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
Defined Benefit Plan, Investment Performance Review Period           3 years    
Pension Plans, Defined Benefit [Member] | Maximum [Member]                
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
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           40.50% 41.30%  
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           15.50% 14.90%  
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           11.20% 11.00%  
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           15.10% 15.20%  
Number of shares of Atmos Energy common stock in Master Trust           956,700 1,169,700  
Amount of dividend income from Atmos Energy common stock in Master Trust           $ 1,800 $ 1,800  
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           17.70% 17.60%  
Pension Plans, Defined Benefit [Member] | Pension Costs [Member] | Unrecognized Transition Obligation [Member]                
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]                
Net Regulatory Assets             $ 0  
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           $ (1,509) (1,735)  
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           127,028 120,948  
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           $ 125,519 $ 119,213  
Pension Account Plan, Defined Benefit [Member]                
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
Defined Benefit Plan, Vesting Period           3 years    
Supplemental Employee Retirement Plan [Member]                
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract                
Discount rate           4.55% 4.43% 4.95%
Rate of compensation increase           3.50% 3.50% 3.50%
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract                
Discount rate           3.73% 4.55%  
Rate of compensation increase           3.50% 3.50%  
Defined Benefit Plan Change In Benefit Obligation Roll Forward                
Accumulated benefit obligation           $ 137,616 $ 118,835  
Benefit obligation at beginning of year           122,393 113,219  
Service cost           2,371 3,971 $ 3,607
Interest cost           5,185 4,943 4,966
Actuarial (gain) loss           17,229 4,811  
Benefits paid           (4,604) (4,551)  
Benefit obligation at end of year           142,574 122,393 113,219
Funded Status of Plan Reconciliation                
Funded status           (142,574) (122,393)  
Unrecognized prior service cost           0 0  
Unrecognized net loss           0 0  
Net amount recognized           (142,574) (122,393)  
Estimated Future Benefits Payment, Total                
FY2017           36,604    
FY2018           14,289    
FY2019           7,181    
FY2020           4,395    
FY2021           4,306    
2022-2026           60,658    
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
Settlements           0 0 $ 4,539
Supplemental Employee Retirement Plan [Member] | Supplemental Executive Benefit Plan Costs [Member] | Unrecognized Transition Obligation [Member]                
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]                
Net Regulatory Assets             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           51,558 36,915  
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           $ 51,558 $ 36,915  
Supplemental Executive Benefit Plan, Defined Benefit [Member]                
Defined Benefit Plan Change In Benefit Obligation Roll Forward                
Benefits paid         $ (16,800)      
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
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]                
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
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%    
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]                
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]                
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract                
Discount rate           4.55% 4.43% 4.95%
Expected return on plan assets           4.45% 4.60% 4.60%
Initial Trend           7.50% 7.50% 8.00%
Ultimate trend rate           5.00% 5.00% 5.00%
Ultimate trend rate reached in           2021 2020 2020
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract                
Discount rate           3.73% 4.55%  
Expected return on plan assets           4.45% 4.45%  
Initial trend rate           7.50% 7.50%  
Ultimate trend rate           5.00% 5.00%  
Ultimate trend reached in           2022 2021  
Defined Benefit Plan Change In Benefit Obligation Roll Forward                
Benefit obligation at beginning of year           $ 267,179 $ 315,118  
Service cost           10,823 15,583 $ 16,784
Interest cost           12,424 14,385 15,951
Plan participants' contributions           4,289 4,563  
Actuarial (gain) loss           (1,052) (69,962)  
Benefits paid           (14,441) (12,508)  
Benefit obligation at end of year           279,222 267,179 $ 315,118
Funded Status of Plan Reconciliation                
Funded status           (120,245) (129,170)  
Unrecognized prior service cost           0 0  
Unrecognized transition obligation           0 0  
Unrecognized net loss           0 0  
Net amount recognized           (120,245) $ (129,170)  
Estimated company benefit payments                
FY2017           15,806    
FY2018           11,602    
FY2019           12,165    
FY2020           13,246    
FY2021           14,210    
2022-2026           84,642    
Estimated retiree benefit payments                
FY2017           3,679    
FY2018           3,992    
FY2019           4,036    
FY2020           4,756    
FY2021           5,420    
2022-2026           36,837    
Disclosure Of Expected Gross Prescription Drug Subsidy Receipts Abstract                
FY2017           0    
FY2018           0    
FY2019           0    
FY2020           0    
FY2021           0    
2022-2026           0    
Estimated Future Benefits Payment, Total                
FY2017           19,485    
FY2018           15,594    
FY2019           16,201    
FY2020           18,002    
FY2021           19,630    
2022-2026           121,479    
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           4,539    
Effect of One Percentage Point Decrease on Service and Interest Cost Components           (3,596)    
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation           42,079    
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation           $ (34,531)    
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [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 Plans, Estimated Future Employer Contributions in Next Fiscal Year           $ 10,000    
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           $ 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           97.20% 97.50%  
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [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           2.80% 2.50%  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Postretirement Benefit Costs [Member] | Unrecognized Transition Obligation [Member]                
Net Periodic Benefit Cost Not Yet Recognized, Recorded as Regulatory Assets [Abstract]                
Net Regulatory Assets             $ 82  
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           $ (2,880) (4,524)  
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           (54,298) (47,149)  
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           $ (57,178) $ (51,591)  
[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.