Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Amount, after accumulated amortization, of debt discount (premium).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28567-108399

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28541-108399

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

+ Details

Name:	us-gaap_DeferredFinanceCostsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Present value of lessee's discounted obligation for lease payments from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918701-209980

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

+ Details

Name:	us-gaap_FinanceLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt and lease obligation, classified as noncurrent.

+ References

+ Details

Name:	us-gaap_LongTermDebtAndCapitalLeaseObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt and lease obligation, classified as current.

+ References

+ Details

Name:	us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt and lease obligation, including portion classified as current.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_UnsecuredSeniorNotesDue2027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_Unsecured2.625SeniorNotesdue2029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_UnsecuredSeniorNotesDue2034Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_UnsecuredSeniorNotesDue2041Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_UnsecuredSeniorNotesDue2043Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_UnsecuredSeniorNotesDue2044Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_Unsecured4.30SeniorNotesdue2048Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_Unsecured4.125SeniorNotesdue2049Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_Unsecured3.375SeniorNotesdue2049Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_Floatingratetermloandue2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_MediumTermNotesDue2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ato_UnsecuredDebenturesDue2028Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Schedule of Long-term Debt Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Oct. 02, 2019	Sep. 30, 2019
Debt Instrument [Line Items]
Long-term debt	$ 4,560,000		$ 3,560,000
Finance lease obligations	8,636		0
Total long-term debt	4,568,636		3,560,000
Original issue (premium) / discount on unsecured senior notes and debentures	623		193
Debt issuance cost	36,515		30,355
Current maturities	157		0
Long-term debt, noncurrent	$ 4,531,341		3,529,452
Unsecured 3.00% Senior Notes, due 2027
Debt Instrument [Line Items]
Interest rate	3.00%
Long-term debt	$ 500,000		500,000
Unsecured 2.625% Senior Notes, due 2029
Debt Instrument [Line Items]
Interest rate	2.625%	2.625%
Long-term debt	$ 300,000		0
Unsecured 5.95% Senior Notes, due 2034
Debt Instrument [Line Items]
Interest rate	5.95%
Long-term debt	$ 200,000		200,000
Unsecured 5.50% Senior Notes, due 2041
Debt Instrument [Line Items]
Interest rate	5.50%
Long-term debt	$ 400,000		400,000
Unsecured 4.15% Senior Notes, due 2043
Debt Instrument [Line Items]
Interest rate	4.15%
Long-term debt	$ 500,000		500,000
Unsecured 4.125% Senior Notes, due 2044
Debt Instrument [Line Items]
Interest rate	4.125%
Long-term debt	$ 750,000		750,000
Unsecured 4.30% Senior Notes, due 2048
Debt Instrument [Line Items]
Interest rate	4.30%
Long-term debt	$ 600,000		600,000
Unsecured 4.125% Senior Notes, due 2049
Debt Instrument [Line Items]
Interest rate	4.125%
Long-term debt	$ 450,000		450,000
Unsecured 3.375% Senior Notes, due 2049
Debt Instrument [Line Items]
Interest rate	3.375%	3.375%
Long-term debt	$ 500,000		0
Floating-rate term loan, due 2022
Debt Instrument [Line Items]
Long-term debt	$ 200,000		0
Medium-term note Series A, 1995-1, 6.67%, due 2025
Debt Instrument [Line Items]
Interest rate	6.67%
Long-term debt	$ 10,000		10,000
Unsecured 6.75% Debentures, due 2028
Debt Instrument [Line Items]
Interest rate	6.75%
Long-term debt	$ 150,000		$ 150,000