Retirement and Post-Retirement Employee Benefit Plans - Schedule of Employee Pension Plans Investments at Fair Value (Details) - Defined Benefit Plan - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Derivatives, Fair Value [Line Items]
Total investments	$ 528,881	$ 530,109	$ 531,691
Fair Value, Recurring
Derivatives, Fair Value [Line Items]
Total investments	528,154	528,798
Fair Value, Recurring | Fair Value, Inputs, Level 1, 2 and 3
Derivatives, Fair Value [Line Items]
Total investments	336,771	355,105
Fair Value, Recurring | Fair Value, Inputs, Level 1, 2 and 3 | Common stocks
Derivatives, Fair Value [Line Items]
Total investments	211,244	212,785
Fair Value, Recurring | Fair Value, Inputs, Level 1, 2 and 3 | Money market funds
Derivatives, Fair Value [Line Items]
Total investments	6,096	16,419
Fair Value, Recurring | Fair Value, Inputs, Level 1, 2 and 3 | Registered investment companies
Derivatives, Fair Value [Line Items]
Total investments	29,762	26,326
Fair Value, Recurring | Fair Value, Inputs, Level 1, 2 and 3 | Mortgage-backed securities
Derivatives, Fair Value [Line Items]
Total investments	15,230	19,986
Fair Value, Recurring | Fair Value, Inputs, Level 1, 2 and 3 | U.S. treasuries
Derivatives, Fair Value [Line Items]
Total investments	21,791	23,815
Fair Value, Recurring | Fair Value, Inputs, Level 1, 2 and 3 | Corporate bonds
Derivatives, Fair Value [Line Items]
Total investments	52,648	55,774
Fair Value, Recurring | Level 1
Derivatives, Fair Value [Line Items]
Total investments	262,761	262,041
Fair Value, Recurring | Level 1 | Common stocks
Derivatives, Fair Value [Line Items]
Total investments	211,244	212,785
Fair Value, Recurring | Level 1 | Money market funds
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 1 | Registered investment companies
Derivatives, Fair Value [Line Items]
Total investments	29,762	26,326
Fair Value, Recurring | Level 1 | Mortgage-backed securities
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 1 | U.S. treasuries
Derivatives, Fair Value [Line Items]
Total investments	21,755	22,930
Fair Value, Recurring | Level 1 | Corporate bonds
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 2
Derivatives, Fair Value [Line Items]
Total investments	74,010	93,064
Fair Value, Recurring | Level 2 | Common stocks
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 2 | Money market funds
Derivatives, Fair Value [Line Items]
Total investments	6,096	16,419
Fair Value, Recurring | Level 2 | Registered investment companies
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 2 | Mortgage-backed securities
Derivatives, Fair Value [Line Items]
Total investments	15,230	19,986
Fair Value, Recurring | Level 2 | U.S. treasuries
Derivatives, Fair Value [Line Items]
Total investments	36	885
Fair Value, Recurring | Level 2 | Corporate bonds
Derivatives, Fair Value [Line Items]
Total investments	52,648	55,774
Fair Value, Recurring | Level 3
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 3 | Common stocks
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 3 | Money market funds
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 3 | Registered investment companies
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 3 | Mortgage-backed securities
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 3 | U.S. treasuries
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Level 3 | Corporate bonds
Derivatives, Fair Value [Line Items]
Total investments	0	0
Fair Value, Recurring | Fair Value Measured at Net Asset Value Per Share | Common/collective trusts
Derivatives, Fair Value [Line Items]
Total investments	122,207	108,975
Fair Value, Recurring | Fair Value Measured at Net Asset Value Per Share | Limited partnerships
Derivatives, Fair Value [Line Items]
Total investments	$ 69,176	$ 64,718