Retirement and Postretirement Employee Benefit Plans - Narrative (Details) $ in Thousands		12 Months Ended
	Jan. 01, 2026	Sep. 30, 2025 USD ($) plan shares	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Recovery period		15 years
Supplemental Executive Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Number of nonqualified supplemental plans offered \| plan		3
Retirement Savings Plan
Defined Benefit Plan Disclosure [Line Items]
New employee contribution percentage		5.00%
Years of service		1 year
Maximum of eligible compensation		65.00%
Employer matching, percent of employee salary		5.00%
Automatic increase feature		1.00%
Deferral rate		10.00%
Percent of employee contribution		100.00%
Fixed annual contribution percent		4.00%
Contribution plan cost		$ 34,300	$ 26,800	$ 23,900
Percent of common stock outstanding		1.10%	1.20%
Retirement Savings Plan \| Forecast
Defined Benefit Plan Disclosure [Line Items]
Employer matching, percent of employee salary	6.00%
Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Vesting period		3 years
Employer contributions		$ 0	$ 5,000
Accounts receivable		$ 5,100	$ 700
Employee Pension Plan \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Medium-term horizon		3 years
Employee Pension Plan \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Medium-term horizon		5 years
Employee Pension Plan \| Company stock
Defined Benefit Plan Disclosure [Line Items]
Shares held by plan (in shares) \| shares		506,700	716,700
Percent of plan assets		14.00%	16.70%
Dividend income		$ 1,900	$ 2,300
Employee Pension Plan \| Retirement Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Employer matching, percent of employee salary		10.00%
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 0	0
Settlement charge		0	(1,529)	(1,030)
Lump sum settlement		0	0
Assets held-in-trust		$ 30,100	31,100
Supplemental Executive Retirement Plans \| Supplemental Executive Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Percent of covered compensation		75.00%
Supplemental Executive Retirement Plans \| 1998 Supplemental Executive Retirement Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Percent of covered compensation		60.00%
Supplemental Executive Retirement Plans \| 2009 Supplemental Executive Retirement Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Vesting period		3 years
Contribution percent		10.00%
Contribution percent for members		25.00%
Attainment age		55 years
Supplemental Executive Retirement Plans \| Executive Officer
Defined Benefit Plan Disclosure [Line Items]
Settlement charge			(1,500)	(1,000)
Lump sum settlement			9,600	$ 5,600
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 0	$ 0
Postretirement Plan \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Expected contribution amount		13,000
Postretirement Plan \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Expected contribution amount		$ 18,000
Postretirement Plan \| Diversified investment funds
Defined Benefit Plan Disclosure [Line Items]
Percent of plan assets		98.60%	97.80%
Maximum investment percent		75.00%

ATMOS ENERGY CORP 10-K