Retirement and Postretirement Employee Benefit Plans - Schedule of Postretirement Benefit Plans Investments at Fair Value (Details) - Postretirement Plan - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 319,384	$ 300,692	$ 255,800
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	319,384	300,692
Fair Value, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	314,956	294,059
Fair Value, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	4,428	6,633
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	4,428	6,633
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	4,428	6,633
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	314,956	294,059
Fair Value, Recurring | Registered investment companies | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	314,956	294,059
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