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Pension Benefit Obligations - Asset Categories (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Post-retirement plan      
Fair values of plan assets $ 772.1 $ 784.9  
Cash and Cash Equivalents      
Post-retirement plan      
Fair values of plan assets 29.5 104.8  
Debt securities      
Post-retirement plan      
Fair values of plan assets 338.3 339.1  
Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 43.5 22.2  
Fixed income funds      
Post-retirement plan      
Fair values of plan assets 26.1 7.9  
Common collective funds      
Post-retirement plan      
Fair values of plan assets 372.0 451.6  
Derivative instruments      
Post-retirement plan      
Fair values of plan assets (37.3) (140.7)  
Quoted Prices in Active Markets (Level 1)      
Post-retirement plan      
Fair values of plan assets 412.0 451.1  
Quoted Prices in Active Markets (Level 1) | Cash and Cash Equivalents      
Post-retirement plan      
Fair values of plan assets 20.2 99.0  
Quoted Prices in Active Markets (Level 1) | Debt securities      
Post-retirement plan      
Fair values of plan assets 338.3 339.1  
Quoted Prices in Active Markets (Level 1) | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 30.3 7.3  
Quoted Prices in Active Markets (Level 1) | Fixed income funds      
Post-retirement plan      
Fair values of plan assets 21.7 5.7  
Quoted Prices in Active Markets (Level 1) | Derivative instruments      
Post-retirement plan      
Fair values of plan assets 1.5    
Significant Other Observable Inputs (Level 2)      
Post-retirement plan      
Fair values of plan assets (11.9) (117.8)  
Significant Other Observable Inputs (Level 2) | Cash and Cash Equivalents      
Post-retirement plan      
Fair values of plan assets 9.3 5.8  
Significant Other Observable Inputs (Level 2) | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 13.2 14.9  
Significant Other Observable Inputs (Level 2) | Fixed income funds      
Post-retirement plan      
Fair values of plan assets 4.4 2.2  
Significant Other Observable Inputs (Level 2) | Derivative instruments      
Post-retirement plan      
Fair values of plan assets (38.8) (140.7)  
Significant Unobservable Inputs (Level 3)      
Post-retirement plan      
Fair values of plan assets     $ 4.0
Investments measured at NAV      
Post-retirement plan      
Fair values of plan assets 372.0 451.6  
Investments measured at NAV | Common collective funds      
Post-retirement plan      
Fair values of plan assets $ 372.0 $ 451.6