Revenue Narrative (Details) $ in Millions	6 Months Ended
Mar. 31, 2019 USD ($)
Alternative Revenue Program [Line Items]
Rate Case Revenue Benchmark	$ 69.4
Regulatory Mechanism Threshold	75.00%

APT has a regulatory mechanism that requires that we share with its tariffed customers a set percentage, as determined through the rate case filings, of the difference between the total non-tariffed revenues earned during a test period and a revenue benchmark.

No definition available.

Name:	ato_RegulatoryMechanismThreshold
Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration