Discontinued Operations - Impact of our discontinued operation cash flow hedges (Details) $ in Thousands	12 Months Ended
Sep. 30, 2017 USD ($)
Discontinued Operations and Disposal Groups [Abstract]
Loss reclassified from AOCI for effective portion of natural gas marketing commodity contracts	$ (2,612)
Gain arising from ineffective portion of natural gas marketing commodity contracts	111
Gain on discontinuance of cash flow hedging of natural gas marketing commodity contracts reclassified from AOCI	10,579
Total impact on purchased gas cost reflected in income from discontinued operations	$ 8,078

The amount of net gains or losses on a cash flow hedge reclassified to purchased gas cost from accumulated other comprehensive income or representing the ineffective portion of the hedge.

No definition available.

+ Details

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- Definition

The amount of net gains or losses on a cash flow hedge reclassified in the period to purchased gas cost from accumulated other comprehensive income

+ References

No definition available.

+ Details

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Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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- References

No definition available.

+ Details

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- Definition

The net gain (loss) recognized in earnings during the reporting period representing the amount of the cash flow hedges' ineffectiveness.

+ References

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 (d)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

The amount of net gain (loss) 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=112277219&loc=d3e80748-113994

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