Retirement and Post-Retirement Employee Benefit Plans - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Recovery period	15 years
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 30,418,000	$ 27,495,000
Assets held-in-trust	38,100,000	41,900,000
Settlement charge	(9,151,000)	(9,180,000)	$ 0
Lump sum settlement	25,698,000	22,729,000
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 20,581,000	16,833,000
Percent of projected claims paid	80.00%
Remaining percent of claims paid by participant	20.00%
Defined Benefit Plan, employer contribution limit	3.00%
Postretirement Plans | Minimum
Defined Benefit Plan Disclosure [Line Items]
Expected contribution amount	$ 15,000,000
Postretirement Plans | Maximum
Defined Benefit Plan Disclosure [Line Items]
Expected contribution amount	$ 25,000,000
Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Vesting period	3 years
Employer contributions	$ 10,000,000	0
Accounts receivable	$ 2,100,000	$ 700,000
Defined Benefit Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Medium-term horizon	3 years
Defined Benefit Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Medium-term horizon	5 years
Company stock | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Shares held by plan (in shares)	716,700
Percent of plan assets	10.60%	13.00%
Dividend income	$ 1,800,000	$ 1,600,000
Diversified investment funds | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Percent of plan assets	97.90%	97.40%
Maximum investment percent	75.00%
Supplemental Executive Benefit Plan | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Percent of covered compensation	75.00%
1998 Supplemental Executive Retirement Plan, Defined Benefit | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Percent of covered compensation	60.00%
2009 Supplemental Executive Retirement Plan, Defined Benefit | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Vesting period	3 years
Contribution percent	10.00%
Contribution percent for members	25.00%
Attainment age	55 years
Retirement Savings Plan
Defined Benefit Plan Disclosure [Line Items]
Maximum of eligible compensation	65.00%
New employee contribution percentage	4.00%
Percent of employee contribution	100.00%
Employer matching, percent of employee salary	4.00%
Years of service	1 year
Fixed annual contribution percent	4.00%
Years of service vesting period	3 years
Contribution plan cost	$ 20,600,000	$ 17,900,000	$ 16,700,000
Percent of common stock outstanding	1.90%	2.20%
Executive Officer | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Settlement charge	$ (9,200,000)
Lump sum settlement	$ 25,700,000