Retirement and Post-Retirement Employee Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Recovery period	15 years
Assets held-in-trust	$ 46,500	$ 42,900
Contribution plan cost	$ 16,200	15,400	$ 15,800
Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Vesting period	3 years
Employer contribution	$ 7,000	5,000
Accounts receivable	$ 2,000	600
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	$ 18,410	13,142
Settlement recognized loss	(4,159)	(2,685)	$ 0
Settlements	13,887	8,554
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 17,419	$ 13,687
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	12.70%	11.80%
Dividend income	$ 1,400	$ 1,700
Diversified investment funds | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Percent of plan assets	97.50%	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
Annual rate	4.69%
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
Percent of common stock outstanding	3.20%	3.70%
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Settlement recognized loss	$ 4,200
Settlements	$ 13,900