Retirement and Post-Retirement Employee Benefit Plans - Schedule of Postretirement Benefit Plans Investments at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	$ 99,385	$ 88,409
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	64,151	57,468
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	35,234	30,941
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	0	0
Postretirement Plans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	199,361	184,790
Postretirement Plans | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	194,358	180,256
Postretirement Plans | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	5,003	4,534
Postretirement Plans | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	0	0
Money market funds | Postretirement Plans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	5,003	4,534
Money market funds | Postretirement Plans | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	0	0
Money market funds | Postretirement Plans | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	5,003	4,534
Money market funds | Postretirement Plans | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	0	0
Registered investment companies | Postretirement Plans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	194,358	180,256
Registered investment companies | Postretirement Plans | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	194,358	180,256
Registered investment companies | Postretirement Plans | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	0	0
Registered investment companies | Postretirement Plans | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	$ 0	$ 0