Anticipated debt issuance amount of future debt issuance.

No definition available.

Name:	ato_AnticipatedDebtIssuanceAmount
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of cash paid during the period on settlement of a hedge.

+ References

No definition available.

+ Details

Name:	ato_DerivativeCashPaidonHedge
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Gain or loss recognized in earnings during the reporting period representing the amount of the hedges' ineffectiveness for fair value and cash flow hedges.

+ References

No definition available.

+ Details

Name:	ato_GainLossOnHedgeIneffectiveness
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Actual percentage of heating season gas purchases hedged using financial instruments.

+ References

No definition available.

+ Details

Name:	ato_HedgePercentageOfActualHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Actual volume of heating season gas purchases hedged using financial instruments.

+ References

No definition available.

+ Details

Name:	ato_HedgeVolumeOfActualHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum anticipated percentage of heating season gas purchases hedged using financial instruments.

+ References

No definition available.

+ Details

Name:	ato_MaximumHedgePercentageOfAnticipatedHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum anticipated percentage of heating season gas purchases hedged using financial instruments.

+ References

No definition available.

+ Details

Name:	ato_MinimumHedgePercentageOfAnticipatedHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The domestic federal statutory tax rate applicable under enacted tax laws to the Nonregulated operations pretax income from continuing operations for the period. The "statutory" tax rate shall be the regular tax rate if there are alternative tax systems.

+ References

No definition available.

+ Details

Name:	ato_NonregulatedSegmentEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The domestic federal statutory tax rate applicable under enacted tax laws to the Regulated operations pretax income from continuing operations for the period. The "statutory" tax rate shall be the regular tax rate if there are alternative tax systems.

+ References

No definition available.

+ Details

Name:	ato_RegulatedSegmentsEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average cost of heating season gas purchased using financial instruments.

+ References

No definition available.

+ Details

Name:	ato_WeightedAverageCostOfActualHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of cash received during the period on settlement of a hedge.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeCashReceivedOnHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

+ Details

Name:	us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of gain reclassified into earnings in the period when cash flow hedge is discontinued because it is probable that the original forecasted transactions will not occur by the end of the original period or an additional two month time period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

+ Details

Name:	us-gaap_GainOnDiscontinuationOfCashFlowHedgeDueToForecastedTransactionProbableOfNotOccurring
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_GeneralDiscussionOfDerivativeInstrumentsAndHedgingActivitiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ato_InterestRateHedgesSeniorNotes2015IssuanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ato_InterestRateHedgesSeniorNotes2017IssuanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ato_InterestRateHedgesSeniorNotes2019IssuanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments Descriptions (Details) $ in Millions	1 Months Ended		12 Months Ended
Financial Instruments Descriptions (Details) $ in Millions	Jun. 30, 2017 USD ($)	Oct. 31, 2014 USD ($)	Sep. 30, 2017 USD ($) $ / Mcf MMcf	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
General Discussion of Derivative Instruments and Hedging Activities [Abstract]
Minimum Hedge Percentage Of Anticipated Heating Season Gas			25.00%
Maximum Hedge Percentage Of Anticipated Heating Season Gas			50.00%
Hedge Percentage Of Actual Heating Season Gas			27.00%
Hedge Volume Of Actual Heating Season Gas \| MMcf			16,200
Weighted Average Cost Of Actual Heating Season Gas \| $ / Mcf			3.08
Gain on Discontinuation of Cash Flow Hedge			$ 10.6
Gain (Loss) On Hedge Ineffectiveness			$ 3.4	$ 21.6	$ 0.2
Regulated Segments Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate			37.00%	37.00%
Nonregulated Segment Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate				39.00%
Gain (Loss) On Derivative Instruments Not Designated Hedges Net Pretax			$ 6.8	$ (15.5)	$ 15.5
Interest Rate Hedges, Senior Notes 2015 Issuance [Member]
Derivative [Line Items]
Derivative, Notional Amount			500.0
Derivative, Cash Received on Hedge		$ 13.4
Interest Rate Hedges, Senior Notes 2017 Issuance [Member]
Derivative [Line Items]
Derivative, Notional Amount			210.0
Derivative, Cash Paid on Hedge	$ 37.0
Anticipated Debt Issuance, Amount			250.0
Interest Rate Hedges, Senior Notes 2019 Issuance [Member]
Derivative [Line Items]
Derivative, Notional Amount			450.0
Anticipated Debt Issuance, Amount			$ 450.0