Retirement and Postretirement Employee Benefit Plans - Schedule of Postretirement Benefit Plans Investments at Fair Value (Details) - Postretirement Plan - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 255,800	$ 229,686	$ 268,199
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	255,800	229,686
Fair Value, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	251,041	224,472
Fair Value, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	4,759	5,214
Fair Value, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0	0
Fair Value, Recurring | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	4,759	5,214
Fair Value, Recurring | Money market funds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0	0
Fair Value, Recurring | Money market funds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	4,759	5,214
Fair Value, Recurring | Money market funds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0	0
Fair Value, Recurring | Registered investment companies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	251,041	224,472
Fair Value, Recurring | Registered investment companies | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	251,041	224,472
Fair Value, Recurring | Registered investment companies | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0	0
Fair Value, Recurring | Registered investment companies | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 0	$ 0