Retirement and Post-Retirement Employee Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Recovery period	15 years
Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Vesting period	3 years
Employer contribution	$ 0	$ 8,500
Accounts receivable	$ 700	1,300
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
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 27,495	8,478
Assets held-in-trust	41,900	44,000
Settlement charge	(9,180)	0	$ (4,159)
Lump sum settlement	22,729	0
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 16,833	$ 13,489
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
Postretirement Plans | Maximum
Defined Benefit Plan Disclosure [Line Items]
Expected contribution amount	$ 25,000
Company stock | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Shares held by Plan (in shares)	716,700	716,700
Percent of plan assets	13.00%	15.40%
Dividend income	$ 1,600	$ 1,500
Diversified investment funds | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Percent of plan assets	97.40%	97.10%
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	$ 17,900	$ 16,700	$ 16,200
Percent of common stock outstanding	2.20%	2.60%
Executive Officer | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Settlement charge	$ 9,200
Lump sum settlement	$ 22,700