Nature of Business (Details) customers in Millions	Sep. 30, 2016 customers regulated_gas_distributions_divisions
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number Of Divisions, Regulated Distribution \| regulated_gas_distributions_divisions	6
Number Of Customers, Regulated Distribution \| customers	3

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