Fair Value Measurements (Details) (USD $)	Mar. 31, 2015	Sep. 30, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	$ (141,605,000)	$ (46,298,000)
Derivative Asset	16,947,000us-gaap_DerivativeAssets	58,865,000us-gaap_DerivativeAssets
Hedged portion of gas stored underground	36,237,000	40,492,000
Available-for-sale Securities	79,862,000us-gaap_AvailableForSaleSecurities	79,613,000us-gaap_AvailableForSaleSecurities
Total assets	133,046,000us-gaap_AssetsFairValueDisclosure	178,970,000us-gaap_AssetsFairValueDisclosure
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(194,328,000)	(72,056,000)
Derivative Liability	138,074,000us-gaap_DerivativeLiabilities	21,856,000us-gaap_DerivativeLiabilities
Additional Fair Value Elements [Abstract]
Derivative, Cash Collateral	52,700,000ato_DerivativeCashCollateral	25,800,000ato_DerivativeCashCollateral
Cash Held In Margin Accounts Classified Current Risk Management Asset	16,600,000	22,700,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	36,100,000	3,100,000
Debt Instrument Carrying Amount	2,460,000,000us-gaap_DebtInstrumentCarryingAmount	2,460,000,000us-gaap_DebtInstrumentCarryingAmount
Debt Instrument Fair Value	2,885,149,000us-gaap_DebtInstrumentFairValue	2,769,541,000us-gaap_DebtInstrumentFairValue
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	151,000	2,185,000
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	46,491,000	44,014,000
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	33,220,000	33,414,000
Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	0	0
Derivative Asset	364,000	36,140,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	0	0
Derivative Liability	138,074,000	21,856,000
Additional Fair Value Elements [Abstract]
Cash Held In Margin Accounts Classified Current Risk Management Asset	0	0
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	0	0
Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(141,605,000)	(46,298,000)
Derivative Asset	16,583,000	22,725,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(194,328,000)	(72,056,000)
Derivative Liability	0	0
Additional Fair Value Elements [Abstract]
Cash Held In Margin Accounts Classified Current Risk Management Asset	16,583,000	22,725,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	36,140,000	3,033,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	7,000	25,000
Hedged portion of gas stored underground	36,237,000	40,492,000
Available-for-sale Securities	46,491,000	44,014,000
Total assets	82,735,000	84,531,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	7,000	12,000
Fair Value Inputs Level 1 [Member] \| Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 1 [Member] \| Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	46,491,000	44,014,000
Fair Value Inputs Level 1 [Member] \| Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 1 [Member] \| Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	0	0
Fair Value Inputs Level 1 [Member] \| Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	7,000	25,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	7,000	12,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	158,545,000	105,138,000
Hedged portion of gas stored underground	0	0
Available-for-sale Securities	33,371,000	35,599,000
Total assets	191,916,000	140,737,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	332,395,000	93,900,000
Fair Value Inputs Level 2 [Member] \| Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	151,000	2,185,000
Fair Value Inputs Level 2 [Member] \| Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 2 [Member] \| Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	33,220,000	33,414,000
Fair Value Inputs Level 2 [Member] \| Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	364,000	36,140,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	138,074,000	21,856,000
Fair Value Inputs Level 2 [Member] \| Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	158,181,000	68,998,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	194,321,000	72,044,000
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	0	0
Hedged portion of gas stored underground	0	0
Available-for-sale Securities	0	0
Total assets	0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	0	0
Fair Value Inputs Level 3 [Member] \| Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 3 [Member] \| Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 3 [Member] \| Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 3 [Member] \| Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	0	0
Fair Value Inputs Level 3 [Member] \| Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	$ 0	$ 0

Amount of cash pledged as collateral against derivative liabilities.

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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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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Amount of right to receive cash collateral under master netting arrangements offset against derivative liabilities.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and right to receive cash collateral under master netting arrangements.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

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