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Retirement Plans - Defined Contribution Retirement Plans - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, employer discretionary contribution amount $ 27.1 $ 23.3 $ 19.2
United States | 401k Plan | First Matching Contributions      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, employer matching contribution, percent of employees' gross pay (as a percent) 100.00%    
Defined contribution plan, employers matching contribution, annual vesting percentage (as a percent) 3.00%    
United States | 401k Plan | Second Matching Contributions      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, employer matching contribution, percent of employees' gross pay (as a percent) 50.00%    
Defined contribution plan, employers matching contribution, annual vesting percentage (as a percent) 4.00%    
United Kingdom      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, employers matching contribution, annual vesting percentage (as a percent) 100.00%    
Minimum | United States | 401k Plan      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, annual contributions per employee, percent (as a percent) 1.00%    
Maximum | United States | 401k Plan      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, annual contributions per employee, percent (as a percent) 80.00%    
Defined contribution plan, employers matching contribution, annual vesting percentage (as a percent) 7.00%    
Maximum | United Kingdom      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, employer matching contribution, percent of employees' gross pay (as a percent) 10.00%