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Defined Contribution Plans - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Retirement Benefits [Abstract]      
Participants annual maximum contribution to defined contribution plan, pre-tax 50.00%    
Percentage of annual bonus eligible for defined contribution to plan 100.00%    
Loss Contingencies [Line Items]      
Plan's employer matching contribution 75.00%    
Expenses recognized for defined contribution plans $ 81.4 $ 59.2 $ 52.2
Other Plan Changes [Member]      
Loss Contingencies [Line Items]      
Plan's employer matching contribution 85.00%