Revenue and Accounts Receivable - Narrative (Details)	12 Months Ended
Sep. 30, 2021
Revenue [Abstract]
Gas costs portion of customer bills	79.00%

Percent Of Customers Whose Gas Cost Portion Of Bill Excluded From Allowance For Uncollectable Account

No definition available.

Name:	ato_PercentOfCustomersWhoseGasCostPortionOfBillExcludedFromAllowanceForUncollectableAccount
Namespace Prefix:	ato_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration