Nature of Business (Details) - Jun. 30, 2015 customers in Millions	state customers regulated_gas_distributions_divisions
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of Customers, Regulated Distribution \| customers	3
Number of Divisions, Regulated Distribution	6
Number of States in which Entity Operates \| state	8

Represents the approximate number of customers in our regulated distribution segment.

No definition available.

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