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Pension Benefit Obligations - Asset Categories (Details) - USD ($)
$ in Millions
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Post-retirement plan      
Fair values of plan assets $ 784.9 $ 1,390.7  
Cash and Cash Equivalents [Member]      
Post-retirement plan      
Fair values of plan assets 104.8 49.8  
Equity and debt securities      
Post-retirement plan      
Fair values of plan assets 339.1 488.5  
Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 22.2 81.2  
Fixed Income Funds [Member]      
Post-retirement plan      
Fair values of plan assets 7.9 29.3  
Common collective funds      
Post-retirement plan      
Fair values of plan assets 451.6 734.8  
Derivative instruments      
Post-retirement plan      
Fair values of plan assets (140.7) 7.1  
Fair Value, Inputs, Level 1 [Member]      
Post-retirement plan      
Fair values of plan assets 451.1 599.1  
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]      
Post-retirement plan      
Fair values of plan assets 99.0 25.1  
Fair Value, Inputs, Level 1 [Member] | Equity and debt securities      
Post-retirement plan      
Fair values of plan assets 339.1 488.5  
Fair Value, Inputs, Level 1 [Member] | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 7.3 60.5  
Fair Value, Inputs, Level 1 [Member] | Fixed Income Funds [Member]      
Post-retirement plan      
Fair values of plan assets 5.7 25.0  
Fair Value, Inputs, Level 2 [Member]      
Post-retirement plan      
Fair values of plan assets (117.8) 52.8  
Fair Value, Inputs, Level 2 [Member] | Cash and Cash Equivalents [Member]      
Post-retirement plan      
Fair values of plan assets 5.8 24.7  
Fair Value, Inputs, Level 2 [Member] | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 14.9 16.7  
Fair Value, Inputs, Level 2 [Member] | Fixed Income Funds [Member]      
Post-retirement plan      
Fair values of plan assets 2.2 4.3  
Fair Value, Inputs, Level 2 [Member] | Derivative instruments      
Post-retirement plan      
Fair values of plan assets (140.7) 7.1  
Fair Value, Inputs, Level 3 [Member]      
Post-retirement plan      
Fair values of plan assets   4.0 $ 3.4
Fair Value, Inputs, Level 3 [Member] | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets   4.0  
Investments measured at NAV      
Post-retirement plan      
Fair values of plan assets 451.6 734.8  
Investments measured at NAV | Common collective funds      
Post-retirement plan      
Fair values of plan assets $ 451.6 $ 734.8