XML 63 R42.htm IDEA: XBRL DOCUMENT v3.25.3

Retirement and Postretirement Employee Benefit Plans (Tables)

12 Months Ended

Sep. 30, 2025

Retirement Benefits [Abstract]

Schedule of Net Periodic Benefit Cost Not yet Recognized

Additionally, the amounts that have not yet been recognized in net periodic pension cost that have been recorded as regulatory assets or liabilities are as follows:

			Employee Pension Plan						Supplemental Executive Retirement Plans						Postretirement Plan						Total
			(In thousands)
September 30, 2025
Unrecognized prior service credit			$	—					$	—					$	(9,984)					$	(9,984)
Unrecognized actuarial (gain) loss			(179,351)						17,661						(116,690)						(278,380)
			$	(179,351)					$	17,661					$	(126,674)					$	(288,364)
September 30, 2024
Unrecognized prior service credit			$	—					$	—					$	(24,897)					$	(24,897)
Unrecognized actuarial (gain) loss			(126,989)						16,136						(111,500)						(222,353)
			$	(126,989)					$	16,136					$	(136,397)					$	(247,250)

Schedule of Allocation of Plan Assets

The following table presents asset allocation information for the Master Trust as of September 30, 2025 and 2024.

			Targeted Allocation  Range						Actual Allocation September 30
Security Class									2025						2024
Domestic equities			35%-55%						43.6%						42.8%
International equities			10%-20%						15.5%						15.8%
Fixed income			5%-45%						24.3%						22.3%
Company stock			0%-15%						14.0%						16.7%
Other assets			0%-20%						2.6%						2.4%

The following table presents asset allocation information for the Retiree Medical Plan assets as of September 30, 2025 and 2024.

			Actual Allocation September 30
Security Class			2025						2024
Diversified investment funds			98.6%						97.8%
Cash and cash equivalents			1.4%						2.2%

Schedule of Assumptions Used

Additional assumptions are presented in the following table:

			Pension Asset												Pension Cost
			2025						2024						2025						2024						2023
Discount rate			5.45		%				5.02		%				5.02		%				6.10		%				5.66		%
Rate of compensation increase			3.50		%				3.50		%				3.50		%				3.50		%				3.50		%
Expected return on plan assets			6.50		%				6.50		%				6.50		%				6.25		%				6.25		%
Interest crediting rate (1)			4.69		%				4.69		%				4.69		%				4.69		%				4.69		%

(1)    The interest crediting rate assumption for the development of the fiscal 2025 pension asset is 4.73% for calendar year 2026, then reverting back to 4.69% for all future years.

These assumptions are presented in the following table:

			Pension Liability												Pension Cost
			2025						2024						2025						2024						2023
Discount rate (1)			5.14		%				4.92		%				4.92		%				5.85		%				5.50		%
Rate of compensation increase			3.50		%				3.50		%				3.50		%				3.50		%				3.50		%
Interest crediting rate (2)			4.69		%				4.69		%				4.69		%				4.69		%				4.69		%

 (1)    Reflects a weighted average discount rate for pension cost for fiscal 2024 and 2023 due to the settlements during the year.

 (2)    The interest crediting rate assumption for the development of the fiscal 2025 pension liability is 4.73% for calendar year 2026, then reverting back to 4.69% for all future years.

The assumptions are presented in the following table:

			Postretirement Asset												Postretirement Cost
			2025						2024						2025						2024						2023
Discount rate			5.47		%				5.01		%				5.01		%				6.06		%				5.61		%
Expected return on plan assets			5.14		%				5.14		%				5.14		%				4.94		%				4.94		%
Initial trend rate			7.00		%				6.75		%				6.75		%				6.50		%				6.25		%
Ultimate trend rate			5.00		%				5.00		%				5.00		%				5.00		%				4.75		%
Ultimate trend reached in			2034						2032						2032						2030						2029

Schedule of Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan

The following table presents the Pension Plan’s accumulated benefit obligation, projected benefit obligation, and funded status as of September 30, 2025 and 2024:

			2025						2024
			(In thousands)
Accumulated benefit obligation			$	428,845					$	448,353
Change in projected benefit obligation:
Benefit obligation at beginning of year			$	470,902					$	431,560
Service cost			10,047						9,546
Interest cost			23,164						25,731
Actuarial (gain) loss			(18,133)						44,205
Benefits paid			(35,556)						(40,140)
Benefit obligation at end of year			450,424						470,902
Change in plan assets:
Fair value of plan assets at beginning of year			595,212						502,412
Actual return on plan assets			64,847						127,940
Employer contributions			—						5,000
Benefits paid			(35,556)						(40,140)
Fair value of plan assets at end of year			624,503						595,212
Reconciliation:
Funded status			174,079						124,310
Unrecognized prior service cost			—						—
Unrecognized net loss			—						—
Net amount recognized			$	174,079					$	124,310

The following table presents the Supplemental Plans’ accumulated benefit obligation, projected benefit obligation, and funded status as of September 30, 2025 and 2024:

			2025						2024
			(In thousands)
Accumulated benefit obligation			$	73,570					$	71,003
Change in projected benefit obligation:
Benefit obligation at beginning of year			$	71,944					$	75,898
Service cost			1,302						55
Interest cost			3,488						4,024
Actuarial loss			2,547						5,853
Benefits paid			(4,761)						(4,285)
Settlements			—						(9,601)
Benefit obligation at end of year			74,520						71,944
Change in plan assets:
Fair value of plan assets at beginning of year			—						—
Employer contribution			—						—
Benefits paid			—						—
Settlements			—						—
Fair value of plan assets at end of year			—						—
Reconciliation:
Funded status			(74,520)						(71,944)
Unrecognized prior service cost			—						—
Unrecognized net loss			—						—
Accrued pension cost			$	(74,520)					$	(71,944)

The following table presents the Retiree Medical Plan’s benefit obligation and funded status as of September 30, 2025 and 2024:

			2025						2024
			(In thousands)
Change in benefit obligation:
Benefit obligation at beginning of year			$	269,391					$	234,004
Service cost			8,132						6,028
Interest cost			13,463						14,034
Plan participants’ contributions			1,993						2,102
Actuarial (gain) loss			(8,733)						31,135
Benefits paid			(19,065)						(17,912)
Plan amendments			1,873						—
Benefit obligation at end of year			267,054						269,391
Change in plan assets:
Fair value of plan assets at beginning of year			300,692						255,800
Actual return on plan assets			21,500						47,857
Employer contributions			—						—
Benefits paid			(2,808)						(2,965)
Fair value of plan assets at end of year			319,384						300,692
Reconciliation:
Funded status			52,330						31,301
Unrecognized transition obligation			—						—
Unrecognized prior service cost			—						—
Unrecognized net loss			—						—
Accrued postretirement cost			$	52,330					$	31,301

Schedule of Net Benefit Costs

Net periodic pension cost for the Pension Plan for fiscal 2025, 2024, and 2023 is presented in the following table.

			Fiscal Year Ended September 30
			2025						2024						2023
			(In thousands)
Components of net periodic pension cost:
Service cost			$	10,047					$	9,546					$	10,805
Interest cost (1)			23,164						25,731						24,924
Expected return on assets (1)			(30,618)						(28,808)						(29,113)
Amortization of prior service credit (1)			—						—						(121)
Recognized actuarial gain (1)			—						(67)						—
Net periodic pension cost			$	2,593					$	6,402					$	6,495

(1)    The components of net periodic cost other than the service cost component are included in the line item other non-operating income in the consolidated statements of comprehensive income or are capitalized on the consolidated balance sheets as a regulatory asset or liability, as described in Note 2 to the consolidated financial statements.

Net periodic pension cost for the Supplemental Plans for fiscal 2025, 2024, and 2023 is presented in the following table.

			Fiscal Year Ended September 30
			2025						2024						2023
			(In thousands)
Components of net periodic pension cost:
Service cost			$	1,302					$	55					$	845
Interest cost (1)			3,488						4,024						4,227
Recognized actuarial loss (1)			1,022						501						691
Settlements (1)			—						1,529						1,030
Net periodic pension cost			$	5,812					$	6,109					$	6,793

(1)    The components of net periodic cost other than the service cost component are included in the line item other non-operating income in the consolidated statements of comprehensive income or are capitalized on the consolidated balance sheets as a regulatory asset or liability, as described in Note 2 to the consolidated financial statements.

Net periodic postretirement cost for the Retiree Medical Plan for fiscal 2025, 2024, and 2023 is presented in the following table.

			Fiscal Year Ended September 30
			2025						2024						2023
			(In thousands)
Components of net periodic postretirement cost:
Service cost			$	8,132					$	6,028					$	6,183
Interest cost (1)			13,463						14,034						13,911
Expected return on assets (1)			(15,325)						(12,511)						(11,215)
Amortization of prior service credit (1)			(13,040)						(13,040)						(13,142)
Recognized actuarial gain (1)			(9,716)						(10,872)						(7,452)
Net periodic postretirement cost			$	(16,486)					$	(16,361)					$	(11,715)

(1)    The components of net periodic cost other than the service cost component are included in the line item other non-operating income in the consolidated statements of comprehensive income or are capitalized on the consolidated balance sheets as a regulatory asset or liability, as described in Note 2 to the consolidated financial statements.

Schedule of Employee Pension Plans Investments at Fair Value

In addition to the assets shown below, the Pension Plan had net accounts receivable of $5.1 million and $0.7 million at September 30, 2025 and 2024, which materially approximates fair value due to the short-term nature of these assets.

			Assets at Fair Value as of September 30, 2025
			Level 1						Level 2						Level 3						Total
			(In thousands)
Investments:
Common stocks			$	284,573					$	—					$	—					$	284,573
Money market funds			—						16,293						—						16,293
Registered investment companies			123,022						—						—						123,022
Government securities:
Mortgage-backed securities			—						26,101						—						26,101
U.S. treasuries			15,021						26						—						15,047
Corporate bonds			—						32,517						—						32,517
Asset-backed securities			—						1,475						—						1,475
Total investments measured at fair value			$	422,616					$	76,412					$	—					499,028
Investments measured at net asset value:
Common/collective trusts (1)																					95,558
Limited partnerships (1)																					24,843
Total investments																					$	619,429

			Assets at Fair Value as of September 30, 2024
			Level 1						Level 2						Level 3						Total
			(In thousands)
Investments:
Common stocks			$	289,301					$	—					$	—					$	289,301
Money market funds			—						14,542						—						14,542
Registered investment companies			90,086						—						—						90,086
Government securities:
Mortgage-backed securities			—						24,383						—						24,383
U.S. treasuries			9,398						27						—						9,425
Corporate bonds			—						31,986						—						31,986
Total investments measured at fair value			$	388,785					$	70,938					$	—					459,723
Investments measured at net asset value:
Common/collective trusts (1)																					111,103
Limited partnerships (1)																					23,665
Total investments																					$	594,491

(1)    The fair value of our common/collective trusts and limited partnerships are measured using the net asset value per share practical expedient. There are no redemption restrictions, redemption notice periods, or unfunded commitments for these investments. The redemption frequency is daily.

Schedule of Expected Benefit Payments

The following benefit payments for our defined benefit plans, which reflect expected future service, as appropriate, are expected to be paid in the following fiscal years:

			Pension Plan						Supplemental Plans
			(In thousands)
2026			$	40,122					$	6,277
2027			39,190						9,627
2028			38,729						28,392
2029			38,798						3,783
2030			38,777						3,615
2031-2035			178,831						31,031

The following benefit payments paid by the Company, retirees, and prescription drug subsidies for our Retiree Medical Plan, which reflect expected future service, as appropriate, are expected to be paid in the following fiscal years.

						Company Payments						Retiree Payments						Subsidy Payments						Total Postretirement Benefits
						(In thousands)
2026						$	18,898					$	2,425					$	—					$	21,323
2027						18,859						2,320						—						21,179
2028						18,783						2,186						—						20,969
2029						19,011						2,112						—						21,123
2030						19,364						2,083						—						21,447
2031-2035						104,216						10,770						—						114,986

Schedule of Postretirement Benefit Plans Investments at Fair Value

The following tables set forth by level, within the fair value hierarchy, the Retiree Medical Plan’s assets at fair value as of September 30, 2025 and 2024. The methods used to determine fair value for the assets held by the Retiree Medical Plan are fully described in Note 2 to the consolidated financial statements.

			Assets at Fair Value as of September 30, 2025
			Level 1						Level 2						Level 3						Total
			(In thousands)
Investments:
Money market funds			$	—					$	4,428					$	—					$	4,428
Registered investment companies			314,956						—						—						314,956
Total investments measured at fair value			$	314,956					$	4,428					$	—					$	319,384

			Assets at Fair Value as of September 30, 2024
			Level 1						Level 2						Level 3						Total
			(In thousands)
Investments:
Money market funds			$	—					$	6,633					$	—					$	6,633
Registered investment companies			294,059						—						—						294,059
Total investments measured at fair value			$	294,059					$	6,633					$	—					$	300,692

X

- Definition

Tabular disclosure of investments of the employee pension plans measured at fair value. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3).

+ References

No definition available.

+ Details

Name:	ato_ScheduleOfEmployeePensionPlansInvestmentsAtFairValueTableTextBlock
Namespace Prefix:	ato_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Tabular disclosure of investments of the postretirement benefit plan measured at fair value. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3).

+ References

No definition available.

+ Details

Name:	ato_ScheduleOfPostretirementBenefitPlansInvestmentsAtFairValueTableTextBlock
Namespace Prefix:	ato_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Tabular disclosure of the change in the benefit obligation, fair value of plan assets, and funded status of pension plans or other employee benefit plans.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Subparagraph (a)(b)(c)
-Paragraph 1
-SubTopic 20
-Topic 715
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Name:	us-gaap_ChangesInProjectedBenefitObligationsFairValueOfPlanAssetsAndFundedStatusOfPlanTableTextBlock
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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

X

- Definition

Tabular disclosure of the major categories of plan assets of pension plans and/or other employee benefit plans. This information may include, but is not limited to, the target allocation of plan assets, the fair value of each major category of plan assets, and the level within the fair value hierarchy in which the fair value measurements fall.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 715
-SubTopic 20
-Subparagraph (d)(5)
-Name Accounting Standards Codification
-Paragraph 1
-Section 50
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Name:	us-gaap_ScheduleOfAllocationOfPlanAssetsTableTextBlock
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Tabular disclosure of assumption used to determine benefit obligation and net periodic benefit cost of defined benefit plan. Includes, but is not limited to, discount rate, rate of compensation increase, expected long-term rate of return on plan assets and interest crediting rate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (k)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Name:	us-gaap_ScheduleOfAssumptionsUsedTableTextBlock
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Tabular disclosure of benefits expected to be paid by pension plans and/or other employee benefit plans in each of the next five fiscal years and in the aggregate for the five fiscal years thereafter.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 715
-SubTopic 20
-Subparagraph (f)
-Name Accounting Standards Codification
-Paragraph 1
-Section 50
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Name:	us-gaap_ScheduleOfExpectedBenefitPaymentsTableTextBlock
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (q)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 715
-SubTopic 20
-Subparagraph (h)
-Name Accounting Standards Codification
-Paragraph 1
-Section 50
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Name:	us-gaap_ScheduleOfNetBenefitCostsTableTextBlock
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Tabular disclosure of the amounts related to pension plans and/or other employee benefit plans in accumulated other comprehensive income or loss that have not yet been recognized as components of net periodic benefit cost, such as the net gain (loss), net prior service cost or credit, and net transition asset or obligation.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 715
-SubTopic 20
-Subparagraph (j)
-Name Accounting Standards Codification
-Paragraph 1
-Section 50
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

Name:	us-gaap_ScheduleOfNetPeriodicBenefitCostNotYetRecognizedTableTextBlock
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:textBlockItemType
Balance Type:	na
Period Type:	duration