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Retirement and Post-Retirement Employee Benefit Plans - Asset Allocation Postretirement Benefit Plan (Details) - Postretirement Plans
Sep. 30, 2019
Sep. 30, 2018
Diversified investment funds    
Defined Benefit Plan Disclosure [Line Items]    
Asset Allocation 97.10% 97.50%
Cash and cash equivalents    
Defined Benefit Plan Disclosure [Line Items]    
Asset Allocation 2.90% 2.50%