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Pension Benefit Obligations - Asset Categories (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Post-retirement plan      
Fair values of plan assets $ 1,295.8 $ 1,198.1  
Cash and cash equivalents      
Post-retirement plan      
Fair values of plan assets 50.6 35.7  
Equity and debt securities      
Post-retirement plan      
Fair values of plan assets 442.3 115.5  
Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 31.5 155.7  
Fixed Income Funds      
Post-retirement plan      
Fair values of plan assets 36.2 36.6  
Common collective funds      
Post-retirement plan      
Fair values of plan assets 707.5 668.7  
Assets held by Insurance Company      
Post-retirement plan      
Fair values of plan assets   26.8  
Derivative instruments      
Post-retirement plan      
Fair values of plan assets 27.7 159.1  
Level 1      
Post-retirement plan      
Fair values of plan assets 498.6 299.8  
Level 1 | Cash and cash equivalents      
Post-retirement plan      
Fair values of plan assets 20.2 21.1  
Level 1 | Equity and debt securities      
Post-retirement plan      
Fair values of plan assets 442.3 115.5  
Level 1 | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 13.0 141.9  
Level 1 | Fixed Income Funds      
Post-retirement plan      
Fair values of plan assets 23.1 21.3  
Level 2      
Post-retirement plan      
Fair values of plan assets 86.3 202.8  
Level 2 | Cash and cash equivalents      
Post-retirement plan      
Fair values of plan assets 30.4 14.6  
Level 2 | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 15.1 13.8  
Level 2 | Fixed Income Funds      
Post-retirement plan      
Fair values of plan assets 13.1 15.3  
Level 2 | Derivative instruments      
Post-retirement plan      
Fair values of plan assets 27.7 159.1  
Level 3      
Post-retirement plan      
Fair values of plan assets 3.4 26.8 $ 45.0
Level 3 | Diversified and equity funds      
Post-retirement plan      
Fair values of plan assets 3.4    
Level 3 | Assets held by Insurance Company      
Post-retirement plan      
Fair values of plan assets   26.8  
Investments measured at NAV      
Post-retirement plan      
Fair values of plan assets 707.5 668.7  
Investments measured at NAV | Common collective funds      
Post-retirement plan      
Fair values of plan assets $ 707.5 $ 668.7