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Retirement and Post-Retirement Employee Benefit Plans - Schedule of Postretirement Benefit Plans Investments at Fair Value (Details) - Postretirement Plans - USD ($)
$ in Thousands
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount $ 201,901 $ 199,361 $ 184,790
Level 1      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 195,929 194,358  
Level 2      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 5,972 5,003  
Level 3      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Money market funds      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 5,972 5,003  
Money market funds | Level 1      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Money market funds | Level 2      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 5,972 5,003  
Money market funds | Level 3      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Registered investment companies      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 195,929 194,358  
Registered investment companies | Level 1      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 195,929 194,358  
Registered investment companies | Level 2      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0 0  
Registered investment companies | Level 3      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Defined Benefit Plan, Plan Assets, Amount $ 0 $ 0