Pension Plan and Retirement Benefits - Defined Benefit Plans - Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Change in Benefit Obligation:
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Nonoperating Retirement Benefit Expense	Nonoperating Retirement Benefit Expense
Defined Benefit Pension Plan
Change in Benefit Obligation:
Projected benefit obligation at beginning of year	$ 231,275	$ 315,466
Service cost	2,689	4,728	$ 5,628
Interest cost	11,548	7,936	7,481
Actuarial gains (losses)	(5,236)	(81,225)
Benefits paid	(14,615)	(15,223)
Administrative expenses	(871)	(407)
Projected benefit obligation at end of year	224,790	231,275	315,466
Change in Plan Assets:
Fair value of plan assets at beginning of year	218,321	297,703
Actual return on assets	11,136	(69,752)
Contribution to plan	6,000	6,000
Benefits paid	(14,615)	(15,223)
Administrative expenses	(871)	(407)
Fair value of plan assets at end of year	219,971	218,321	297,703
Funded Status of Plan:
Unfunded status	(4,819)	(12,954)
Defined Benefit Other Postretirement Benefit Plan
Change in Benefit Obligation:
Projected benefit obligation at beginning of year	64,037	82,964
Service cost	1,389	1,825	1,095
Interest cost	3,199	2,234	2,292
Actuarial gains (losses)	(14,171)	(20,343)
Benefits paid	(2,128)	(2,643)
Projected benefit obligation at end of year	52,326	64,037	$ 82,964
Change in Plan Assets:
Contribution to plan	2,128	2,643
Benefits paid	(2,128)	(2,643)
Funded Status of Plan:
Unfunded status	$ (52,326)	$ (64,037)