Retirement and Post-Retirement Employee Benefit Plans - Schedule of Postretirement Benefit Plans Investments at Fair Value (Details) - Postretirement Plans - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 208,245	$ 201,901	$ 199,361
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	208,245	201,901
Fair Value, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	202,720	195,929
Fair Value, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	5,525	5,972
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	5,525	5,972
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	5,525	5,972
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	202,720	195,929
Fair Value, Recurring | Registered investment companies | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	202,720	195,929
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