Nature of Business (Details)	Mar. 31, 2015 state regulated_gas_distributions_divisions customers
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of Customers, Regulated Distribution	3,000,000
Number of Divisions, Regulated Distribution	6
Number of States in which Entity Operates	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