Nature of Business (Details) customer in Millions	Sep. 30, 2020 customer regulated_distribution_division
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of customers serviced \| customer	3,000,000
Number of regulated distribution divisions \| regulated_distribution_division	6

Represents the number of regulated distribution divisions.

No definition available.

Name:	ato_NumberOfDivisionsRegulatedDistribution
Namespace Prefix:	ato_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant