Commitments and Contingencies Commitments and Contingencies (Details) - Inventories Under Indexed Contracts [Member] - Mid-Tex Division [Member]	Jun. 30, 2017 MMcf
Long-term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required Within One Year	53,200
Long Term Purchase Commitment Minimum Quantity Required One To Three Years	37,600
Long Term Purchase Commitment Minimum Quantity Required After Three Years	400

Volume that must be purchased within one to three years following the balance sheet date to avoid penalties or unfavorable effects on pricing, among other adverse consequences.

No definition available.

Name:	ato_LongTermPurchaseCommitmentMinimumQuantityRequiredOneToThreeYears
Namespace Prefix:	ato_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant