Fair Value Measurements (Details) (USD $)	Jun. 30, 2011	Sep. 30, 2010
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value	$ 44,045,000	$ 41,466,000
Fair Value Abstract
Cash Collateral Right To Reclaim Cash	15,400,000	24,900,000
Nonrecurring fair value measurement	6,000,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	4,400,000	12,600,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	11,000,000	12,300,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	3,696,000	18,544,000
Total financial instruments	3,696,000	18,544,000
Hedged portion of gas stored underground	86,544,000	57,507,000
Fair value	44,045,000	41,466,000
Total assets	134,285,000	117,517,000
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	10,645,000	41,430,000
Total liabilities	10,645,000	41,430,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	2,739,000	2,266,000
Nonregulated Segement Derivative Assets	34,367,000	42,462,000
Total financial instruments	37,106,000	44,728,000
Hedged portion of gas stored underground	0	0
Fair value	0	0
Total assets	37,106,000	44,728,000
Natural gas distribution segment	5,263,000	51,866,000
Nonregulated Segement Derivative Liabilities	38,827,000	31,950,000
Total liabilities	44,090,000	83,816,000
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	0	0
Total financial instruments	0	0
Hedged portion of gas stored underground	0	0
Fair value	0	0
Total assets	0	0
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	0	0
Total liabilities	0	0
Netting And Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	(25,006,000)	(41,760,000)
Total financial instruments	(25,006,000)	(41,760,000)
Hedged portion of gas stored underground	0	0
Fair value	0	0
Total assets	(25,006,000)	(41,760,000)
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	(40,388,000)	(66,649,000)
Total liabilities	$ (40,388,000)	$ (66,649,000)

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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- Definition

The amount of the right to reclaim cash collateral under master netting arrangements that has been offset against net derivative instrument positions.

+ Details

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Period Type:	instant

- Definition

ash held in margin accounts classified as current risk managment assets

+ Details

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Period Type:	instant

- Definition

ash held in margin accounts used to offset current risk management liabilites under master netting agreements

+ Details

Name:	ato_CashHeldInMarginAccountsOffsetCurrentRiskManagementLiabilities
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- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ Details

Name:	ato_DerivativeAssetsFairValue
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- Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

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- Definition

Represents gas gathering assets reported on the balance sheet at period end measured at fair value on a nonrecurring basis.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Fair value amount as of the balance sheet date of the hedged portion of natural gas stored underground in depleted gas reservoirs, aquifers, or salt caverns to meet seasonal and peak load demands, and also as insurance against unforeseen supply disruptions, and deemed to be a current asset because it is expected to be used within twelve months or in the normal operating cycle.

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- Definition

This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ Details

Name:	ato_LiabilitiesFairValueDisclosure
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- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, for the natural gas distribution segment.

+ Details

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- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, for the natural gas distribution segment.

+ Details

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- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, for the nonregulated segment.

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- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, for the nonregulated segment.

+ Details

Name:	ato_NonregulatedSegementDerivativeLiabilities
Namespace Prefix:	ato
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- Definition

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. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph b

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