Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Mid-Tex Division [Member]
Long Term Purchase Commitment [Line Items]
Significant Purchase Commitment Amount	$ 46.5	$ 30.4
Nonregulated Segment [Member]
Long Term Purchase Commitment [Line Items]
Significant Purchase Commitment Amount	$ 383.0	$ 350.2
Inventories Under Indexed Contracts [Member] \| Mid-Tex Division [Member]
Long Term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required Within One Year	47,000
Long Term Purchase Commitment Minimum Quantity Required One To Three Years	57,700
Inventories Under Indexed Contracts [Member] \| Nonregulated Segment [Member]
Long Term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required Within One Year	93,200
Long Term Purchase Commitment Minimum Quantity Required One To Three Years	11,600
Long Term Purchase Commitment Minimum Quantity Required After Three Years	400
Inventories Under Fixed Price Contracts [Member]
Long Term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required Within One Year	5,100
Inventories Under Fixed Price Contracts [Member] \| Nonregulated Segment [Member]
Long Term Purchase Commitment [Line Items]
Purchase Commitment Amount Minimum	1.22
Purchase Commitment Amount Maximum	4.49

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

The maximum per-unit amount the entity agreed to spend under fixed price purchase commitment arrangements.

+ References

No definition available.

+ Details

Name:	ato_PurchaseCommitmentAmountMaximum
Namespace Prefix:	ato_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

- Definition

The minimum per-unit amount the entity agreed to spend under fixed price purchase commitment arrangements.

+ References

No definition available.

+ Details

Name:	ato_PurchaseCommitmentAmountMinimum
Namespace Prefix:	ato_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

- Definition

Purchases made during the period under arrangements in which the entity has agreed to expend funds to procure goods or services from one or more suppliers.

+ References

No definition available.

+ Details

Name:	ato_SignificantPurchaseCommitmentAmount
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_LongTermPurchaseCommitmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=ato_InventoriesUnderFixedPriceContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=ato_InventoriesUnderIndexedContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ato_MidTexDivisionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ato_NonregulatedSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: