Retirement and Post-Retirement Employee Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
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	$ 8,500	$ 7,000
Accounts receivable	$ 1,300	2,000
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
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 8,478	18,410
Assets held-in-trust	44,000	46,500
Settlement recognized loss	0	(4,159)	$ (2,685)
Settlements	0	13,887
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 13,489	$ 17,419
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	$ 10,000
Postretirement Plans | Maximum
Defined Benefit Plan Disclosure [Line Items]
Expected contribution amount	$ 20,000
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	15.40%	12.70%
Dividend income	$ 1,500	$ 1,400
Diversified investment funds | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Percent of plan assets	97.10%	97.50%
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	$ 16,700	$ 16,200	$ 15,400
Percent of common stock outstanding	2.60%	3.20%