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

Gain (loss) recognized in earnings during the reporting period representing the amount of the fair value hedges' ineffectiveness due to a difference in timing.

No definition available.

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

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109977999&loc=d3e76258-113986

Name:	us-gaap_ChangeInUnrealizedGainLossOnFairValueHedgingInstruments1
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

Name:	us-gaap_ChangeInUnrealizedGainLossOnHedgedItemInFairValueHedge1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

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

Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959