Retirement and Post-Retirement Employee Benefit Plans Defined Benefit Plans Disclosure (Details) (USD $)	12 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Curtailment loss	$ (162,000)	$ 0
Pension Plans Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	5.39%	5.52%	7.57%
Rate of compensation increase	4.00%	4.00%	4.00%
Expected return on plan assets	8.25%	8.25%	8.25%
Unrecognized transition obligation	0	0
Unrecognized prior service cost	373,000	842,000
Unrecognized actuarial loss	182,486,000	159,539,000
Total unrecognized in net periodic cost	182,113,000	158,697,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	5.05%	5.39%
Rate of compensation increase	3.50%	4.00%
Expected return on plan assets	7.75%	8.25%
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year	407,536,000	380,045,000
Service cost	14,384,000	13,499,000	12,951,000
Interest cost	22,264,000	20,870,000	24,060,000
Actuarial (gain) loss	12,944,000	19,809,000
Benefits paid	(27,534,000)	(26,687,000)
Benefit obligation at end of year	429,432,000	407,536,000	380,045,000
Funded Status Reconciliation [Abstract]
Funded status	(149,228,000)	(105,828,000)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	(149,228,000)	(105,828,000)
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	40.40%	44.10%
International Equities Actual Allocation	13.60%	14.40%
Fixed Income Actual Allocation	21.30%	19.00%
Company Stock Actual Allocation	13.50%	11.30%
Other Actual Allocation	11.20%	11.20%
Estimated Future Benefits Payment, Total
Expected future benefit payments in year one, total	35,286,000
Expected future benefit payments in year two, total	33,109,000
Expected future benefit payments in year three, total	31,753,000
Expected future benefit payments in year four, total	30,633,000
Expected future benefit payments in year five, total	30,648,000
Expected future benefit payments in five fiscal years thereafter, total	146,923,000
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Accumulated benefit obligation	414,489,000	391,915,000
Fair value of accounts receivable	400,000	100,000
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	5.39%	5.52%	7.57%
Expected return on plan assets		5.00%	5.00%
Initial trend	8.00%	7.50%	8.00%
Ultimate trend rate	5.00%	5.00%	5.00%
Ultimate trend rate reached in	2016	2015	2015
Unrecognized transition obligation	3,220,000	4,731,000
Unrecognized prior service cost	8,861,000	10,311,000
Unrecognized actuarial loss	47,540,000	25,694,000
Total unrecognized in net periodic cost	41,899,000	20,114,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	5.05%	5.39%
Expected return on plan assets	5.00%	5.00%
Initial trend	8.00%	8.00%
Ultimate trend rate	5.00%	5.00%
Ultimate trend rate reached in	2018	2016
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year	228,234,000	209,732,000
Service cost	14,403,000	13,439,000	11,786,000
Interest cost	12,813,000	12,071,000	14,080,000
Plan participants' contributions	2,892,000	2,734,000
Actuarial (gain) loss	17,966,000	2,980,000
Benefits paid	(13,046,000)	(12,722,000)
Subsidy Payments	432,000	0
Benefit obligation at end of year	263,694,000	228,234,000	209,732,000
Funded Status Reconciliation [Abstract]
Funded status	(210,629,000)	(175,201,000)
Unrecognized prior service cost	0	0
Unrecognized transition obligation	0	0
Unrecognized net loss	0	0
Net amount recognized	(210,629,000)	(175,201,000)
Actual Allocation Abstract
Diversified investment funds	96.80%	97.50%
Cash and cash equivalents	3.20%	2.50%
Estimated company benefit payments
Estimated company benefit payments in year one	31,519,000
Estimated company benefit payments in year two	13,272,000
Estimated company benefit payments in year three	15,271,000
Estimated company benefit payments in year four	16,789,000
Estimated company benefit payments in year five	18,333,000
Estimated company benefit payments in five fiscal years thereafter	99,139,000
Estimated retiree benefit payments
Estimated retiree benefit payments in year one	3,293,000
Estimated retiree benefit payments in year two	3,895,000
Estimated retiree benefit payments in year three	4,491,000
Estimated retiree benefit payments in year four	5,026,000
Estimated retiree benefit payments in year five	5,672,000
Estimated retiree benefit payments in five fiscal years thereafter	38,238,000
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	34,812,000
Expected future benefit payments in year two, total	17,167,000
Expected future benefit payments in year three, total	19,762,000
Expected future benefit payments in year four, total	21,815,000
Expected future benefit payments in year five, total	24,005,000
Expected future benefit payments in five fiscal years thereafter, total	137,377,000
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	4,155,000
Effect of One Percentage Point Decrease on Service and Interest Cost Components	(3,479,000)
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	30,159,000
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(25,578,000)
Supplemental Executive Retirement Plans, Defined Benefit [Member]
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Net Periodic Benefit Cost Abstract
Discount rate	5.39%	5.52%	7.57%
Rate of compensation increase	4.00%	4.00%
Unrecognized transition obligation	0	0
Unrecognized prior service cost	0	0
Unrecognized actuarial loss	30,654,000	30,753,000
Total unrecognized in net periodic cost	30,654,000	30,753,000
Defined Benefit Plan Weighted Average Assumptions Used In Calculating Benefit Obligation Abstract
Discount rate	5.05%	5.39%
Rate of compensation increase	3.50%	4.00%
Defined Benefit Plan Change In Benefit Obligation Roll Forward
Benefit obligation at beginning of year	108,919,000	102,747,000
Service cost	2,768,000	2,476,000	1,985,000
Interest cost	5,825,000	5,224,000	6,056,000
Actuarial (gain) loss	2,140,000	3,043,000
Benefits paid	(7,537,000)	(4,571,000)
Benefit obligation at end of year	112,115,000	108,919,000	102,747,000
Funded Status Reconciliation [Abstract]
Funded status	(112,115,000)	(108,919,000)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	(112,115,000)	(108,919,000)
Accumulated Benefit Obligation in Excess of Plan Assets
Projected benefit obligation	112,115,000	108,919,000
Accumulated benefit obligation	104,363,000	99,673,000
Fair value of plan assets	0	0
Estimated Future Benefits Payment, Total
Expected future benefit payments in year one, total	25,116,000
Expected future benefit payments in year two, total	6,910,000
Expected future benefit payments in year three, total	4,738,000
Expected future benefit payments in year four, total	6,862,000
Expected future benefit payments in year five, total	4,622,000
Expected future benefit payments in five fiscal years thereafter, total	43,625,000
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Accumulated benefit obligation	$ 104,363,000	$ 99,673,000

The amount of dividend income from employer securities that are included in plan assets.

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An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

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The percentage of the fair value of cash and cash equivalents to the fair value of total plan assets held as of the measurement date.

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The percentage of the fair value of diversified investment funds to the fair value of total plan assets held as of the measurement date.

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The percentage of the fair value of domestic equity securities to the fair value of total plan assets held as of the measurement date.

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The amount of the aggregate postretirement benefits estimated to be paid in the five fiscal years thereafter (the aggregate amount of postretirement benefits estimated to be paid in years 6 through 10 after the date of the latest statement of financial position) by the Company.

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The amount of the postretirement benefits estimated to be paid in Year 5 by the Company.

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The amount of the postretirement benefits estimated to be paid in Year 4 by the Company.

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The amount of the postretirement benefits estimated to be paid in Year 1 by the Company.

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The amount of the postretirement benefits estimated to be paid in Year 3 by the Company.

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The amount of the postretirement benefits estimated to be paid in Year 2 by the Company.

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The amount of the postretirement benefits estimated to be paid in Year 5 by the retirees.

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The amount of the postretirement benefits estimated to be paid in Year 4 by the retirees.

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The amount of the postretirement benefits estimated to be paid in Year 1 by the retirees.

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The amount of the postretirement benefits estimated to be paid in Year 3 by the retirees.

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The amount of the postretirement benefits estimated to be paid in Year 2 by the retirees.

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The percentage of the fair value of fixed income to the fair value of total plan assets held as of the measurement date.

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The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

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The percentage of the fair value of international equity securities to the fair value of total plan assets held as of the measurement date.

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Target allocation maximum percentage in other than debt securities, equity securities, and real estate to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

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Target allocation minimum percentage in other than debt securities, equity securities, and real estate to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

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The ultimate trend rate for health care costs.

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The ultimate trend rate for health care costs.

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The amount of net actuarial (gain) loss that is not yet recognized in net periodic cost and that is recorded in regulatory asset.

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The amount of unrecognized prior service cost that is not yet recognized in net periodic cost and that is recorded in regulatory asset.

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The amount of transition obligation that is not yet recognized in net periodic cost and that is recorded in regulatory asset.

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The year when the ultimate health care cost trend rate is expected to be reached.

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The year when the ultimate health care cost trend rate is expected to be reached.

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The total amounts in regulatory assets that have not been recognized in net periodic pension cost.

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The amounts in regulatory assets related to gains and losses that have not been recognized in net periodic pension cost.

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The amounts in regulatory assets related to prior service cost or credit that have not been recognized in net periodic pension cost.

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The amounts in regulatory assets related to gains and losses that have not been recognized in net periodic pension cost.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. Amounts due from customers or clients for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, as well as any other amounts due from others, including related parties, which may be included in accounts receivable as categorized on the statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 3

-Subparagraph a, b

-Article 5

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For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph e

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- Definition

The net increase or decrease of changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 106

-Paragraph 518

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

-Paragraph 264

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph a

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- Definition

The aggregate net amount recognized in the balance sheet associated with the defined benefit plan(s). Will normally be the same as the Defined Benefit Plan, Funded Status of Plan, Total.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph c

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- Definition

The interest rate used to adjust for the time value of money for the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph j

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- Definition

Expected rate of compensation increases (for pay-related plans).

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph j

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The interest rate used to adjust for the time value of money.

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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-Publisher FASB

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-Subparagraph j

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- Definition

Expected rate of compensation increases (for pay-related plans).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph j

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- Definition

1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. The projected benefit obligation is measured using assumptions as to future compensation levels if the pension benefit formula is based on those future compensation levels (pay-related, final-pay, final-average-pay, or career-average-pay plans). For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date. Prior to an employee's full eligibility date, the accumulated postretirement benefit obligation as of a particular date for an employee is the portion of the expected postretirement benefit obligation attributed to that employee's service rendered to that date; on and after the full eligibility date, the accumulated and expected postretirement benefit obligations for an employee are the same.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph a

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 6

-Subparagraph a

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph E1

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- Definition

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph a, b

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 106

-Paragraph 518

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

-Paragraph 264

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS106-2

-Paragraph 22

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

The amount of contributions made by plan participants. Under a contributory plan active employees or retirees contribute part of the cost. In some contributory plans, those wishing to be covered must contribute; in others, the contributions result in increased benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph b

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- Definition

The amount of decrease that is related to an event that significantly reduces the expected years of future service of present employees or eliminates for a significant number of employees the accrual of defined benefits for some or all of their future services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 88

-Paragraph 6

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 106

-Paragraph 518

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

-Paragraph 264

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph a

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- Definition

The effect of a one-percentage-point decrease in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. (For purposes of this disclosure, all other assumptions shall be held constant, and the effects shall be measured based on the substantive plan that is the basis for the accounting.)

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph m

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- Definition

The effect of a one-percentage- point decrease in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs. (For purposes of this disclosure, all other assumptions shall be held constant, and the effects shall be measured based on the substantive plan that is the basis for the accounting.)

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph m

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- Definition

The effect of a one-percentage-point increase in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. (For purposes of this disclosure, all other assumptions shall be held constant, and the effects shall be measured based on the substantive plan that is the basis for the accounting.)

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph m

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- Definition

The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs. (For purposes of this disclosure, all other assumptions shall be held constant, and the effects shall be measured based on the substantive plan that is the basis for the accounting.)

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph m

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- Definition

The percentage of the fair value of equity securities to the fair value of total plan assets held as of the measurement date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph d(1)

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- Definition

The amount of the aggregate benefits expected to be paid in the five fiscal years thereafter (the aggregate amount of benefits expected to be paid in years 6 through 10 after the date of the latest statement of financial position).

+ References