Retirement and Postretirement Employee Benefit Plans - Narrative (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2024 USD ($) plan shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 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	4.00%
Years of service	1 year
Maximum of eligible compensation	65.00%
Employer matching, percent of employee salary	4.00%
Automatic increase feature	1.00%
Deferral rate	10.00%
Percent of employee contribution	100.00%
Fixed annual contribution percent	4.00%
Contribution plan cost	$ 26,800	$ 23,900	$ 21,900
Percent of common stock outstanding	1.20%	1.40%
Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Vesting period	3 years
Employer contributions	$ 5,000	$ 8,000
Accounts receivable	$ 700	$ 400
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	716,700	716,700
Percent of plan assets	16.70%	15.10%
Dividend income	$ 2,300	$ 2,100
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	(1,529)	(1,030)	$ 0
Lump sum settlement	0	0
Assets held-in-trust	$ 31,100	31,500
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	10,000
Postretirement Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Expected contribution amount	$ 15,000
Postretirement Plan | Diversified investment funds
Defined Benefit Plan Disclosure [Line Items]
Percent of plan assets	97.80%	98.20%
Maximum investment percent	75.00%