Financial Instruments Commodity Contract Volume (Details)	Mar. 31, 2018 MMcf
Nondesignated [Member]
Commodity Contract Outstanding Volumes [Line Items]
Investment Contract Volume	6,251

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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Namespace Prefix:	ato_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration