Retirement and Post-Retirement Employee Benefit Plans Defined Benefit Plans Disclosure (Details) (USD $)	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Unrecognized transition obligation	$ 1,709,000	$ 3,220,000
Unrecognized prior service cost	7,643,000	9,234,000
Unrecognized actuarial loss	294,447,000	260,680,000
Total unrecognized in net periodic cost	288,513,000	254,666,000
Pension Plans Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	5.05%	5.39%	5.52%
Rate of compensation increase	3.50%	4.00%	4.00%
Expected return on plan assets	7.75%	8.25%	8.25%
Unrecognized transition obligation	0	0
Unrecognized prior service cost	232,000	373,000
Unrecognized actuarial loss	187,050,000	182,486,000
Total unrecognized in net periodic cost	186,818,000	182,113,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	4.04%	5.05%
Rate of compensation increase	3.50%	3.50%
Expected return on plan assets	7.75%	7.75%
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year	429,432,000	407,536,000
Service cost	15,084,000	14,384,000	13,499,000
Interest cost	21,568,000	22,264,000	20,870,000
Actuarial (gain) loss	46,197,000	12,944,000
Benefits paid	(24,553,000)	(27,534,000)
Divestitures	(7,697,000)	0
Curtailment loss	0	(162,000)
Benefit obligation at end of year	480,031,000	429,432,000	407,536,000
Funded Status Reconciliation [Abstract]
Funded status	(136,887,000)	(149,228,000)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	(136,887,000)	(149,228,000)
Defined Benefit Plan Information About Plan Assets Abstract
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
Targeted Allocation Range Abstract
Domestic Equities Targeted Allocation Range Minimum	35.00%
Domestic Equities Targeted Allocation Range Maximum	55.00%
International Equities Targeted Allocation Range Minimum	10.00%
International Equities Targeted Allocation Range Maximum	20.00%
Fixed Income Targeted Allocation Range Minimum	10.00%
Fixed Income Targeted Allocation Range Maximum	30.00%
Company Stock Targeted Allocation Range Minimum	5.00%
Company Stock Targeted Allocation Range Maximum	15.00%
Other Targeted Allocation Range Minimum	5.00%
Other Targeted Allocation Range Maximum	15.00%
Actual Allocation Abstract
Domestic Equities Actual Allocation	42.60%	40.40%
International Equities Actual Allocation	13.90%	13.60%
Fixed Income Actual Allocation	18.60%	21.30%
Company Stock Actual Allocation	12.00%	13.50%
Other Actual Allocation	12.90%	11.20%
Estimated Future Benefits Payment, Total
Expected future benefit payments in year one, total	38,800,000
Expected future benefit payments in year two, total	35,551,000
Expected future benefit payments in year three, total	33,953,000
Expected future benefit payments in year four, total	33,536,000
Expected future benefit payments in year five, total	32,740,000
Expected future benefit payments in five fiscal years thereafter, total	156,231,000
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Accumulated benefit obligation	468,440,000	414,489,000
Fair value of accounts receivable	500,000	400,000
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	5.05%	5.39%	5.52%
Expected return on plan assets	5.00%	5.00%	5.00%
Initial trend	8.00%	8.00%	7.50%
Ultimate trend rate	5.00%	5.00%	5.00%
Ultimate trend rate reached in	2018	2016	2015
Unrecognized transition obligation	1,709,000	3,220,000
Unrecognized prior service cost	7,411,000	8,861,000
Unrecognized actuarial loss	63,402,000	47,540,000
Total unrecognized in net periodic cost	57,700,000	41,899,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	4.04%	5.05%
Expected return on plan assets	4.70%	5.00%
Initial trend	8.00%	8.00%
Ultimate trend rate	5.00%	5.00%
Ultimate trend rate reached in	2019	2018
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year	263,694,000	228,234,000
Service cost	16,353,000	14,403,000	13,439,000
Interest cost	13,861,000	12,813,000	12,071,000
Plan participants' contributions	3,649,000	2,892,000
Actuarial (gain) loss	28,815,000	17,966,000
Benefits paid	(13,197,000)	(13,046,000)
Divestitures	(4,860,000)	0
Subsidy Payments	0	432,000
Benefit obligation at end of year	308,315,000	263,694,000	228,234,000
Funded Status Reconciliation [Abstract]
Funded status	(231,243,000)	(210,629,000)
Unrecognized prior service cost	0	0
Unrecognized transition obligation	0	0
Unrecognized net loss	0	0
Net amount recognized	(231,243,000)	(210,629,000)
Actual Allocation Abstract
Diversified investment funds	97.00%	96.80%
Cash and cash equivalents	3.00%	3.20%
Estimated company benefit payments
Estimated company benefit payments in year one	28,317,000
Estimated company benefit payments in year two	15,174,000
Estimated company benefit payments in year three	17,349,000
Estimated company benefit payments in year four	19,221,000
Estimated company benefit payments in year five	20,520,000
Estimated company benefit payments in five fiscal years thereafter	107,055,000
Estimated retiree benefit payments
Estimated retiree benefit payments in year one	3,696,000
Estimated retiree benefit payments in year two	4,487,000
Estimated retiree benefit payments in year three	5,251,000
Estimated retiree benefit payments in year four	6,128,000
Estimated retiree benefit payments in year five	7,083,000
Estimated retiree benefit payments in five fiscal years thereafter	48,114,000
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	32,013,000
Expected future benefit payments in year two, total	19,661,000
Expected future benefit payments in year three, total	22,600,000
Expected future benefit payments in year four, total	25,349,000
Expected future benefit payments in year five, total	27,603,000
Expected future benefit payments in five fiscal years thereafter, total	155,169,000
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	1,426,000
Effect of One Percentage Point Decrease on Service and Interest Cost Components	(1,287,000)
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	21,736,000
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(18,866,000)
Supplemental Executive Retirement Plans, Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	5.05%	5.39%	5.52%
Rate of compensation increase	3.50%	4.00%	4.00%
Unrecognized transition obligation	0	0
Unrecognized prior service cost	0	0
Unrecognized actuarial loss	43,995,000	30,654,000
Total unrecognized in net periodic cost	43,995,000	30,654,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	4.04%	5.05%
Rate of compensation increase	3.50%	3.50%
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year	112,115,000	108,919,000
Service cost	2,108,000	2,768,000	2,476,000
Interest cost	5,142,000	5,825,000	5,224,000
Actuarial (gain) loss	15,459,000	2,140,000
Benefits paid	(4,638,000)	(7,537,000)
Benefit obligation at end of year	130,186,000	112,115,000	108,919,000
Funded Status Reconciliation [Abstract]
Funded status	(130,186,000)	(112,115,000)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	(130,186,000)	(112,115,000)
Estimated Future Benefits Payment, Total
Expected future benefit payments in year one, total	31,108,000
Expected future benefit payments in year two, total	13,453,000
Expected future benefit payments in year three, total	7,658,000
Expected future benefit payments in year four, total	4,680,000
Expected future benefit payments in year five, total	7,385,000
Expected future benefit payments in five fiscal years thereafter, total	41,830,000
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Accumulated benefit obligation	$ 121,815,000	$ 104,363,000