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Fair Value Measurements (Details) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2020
Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
ARO Trust investments $ 235.5 $ 260.5
Additional Fair Value Elements [Abstract]    
Tangible Asset Impairment Charges 170.1  
Quoted Prices In Active Markets for Identical Assets (Level 1)    
Additional Fair Value Elements [Abstract]    
Long-term debt. including current portion 0.0 0.0
Significant Other Oberservable Inputs (Level 2)    
Additional Fair Value Elements [Abstract]    
Long-term debt. including current portion (6,949.0) (6,849.8)
Significant Unobservable Inputs (Level 3)    
Additional Fair Value Elements [Abstract]    
Long-term debt. including current portion 0.0 0.0
Carrying Amount    
Additional Fair Value Elements [Abstract]    
Long-term debt. including current portion (5,239.8) (5,294.6)
Fair Value    
Additional Fair Value Elements [Abstract]    
Long-term debt. including current portion (6,949.0) (6,849.8)
Fair Value, Recurring | Quoted Prices In Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
ARO Trust investments 235.5 260.5
Fair Value, Recurring | Significant Other Oberservable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
ARO Trust investments 0.0 0.0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
ARO Trust investments 0.0 0.0
Fair Value, Recurring | Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
ARO Trust investments 235.5 260.5
Fair Value, Recurring | Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
ARO Trust investments 235.5 $ 260.5
Fair Value, Nonrecurring | Significant Unobservable Inputs (Level 3)    
Additional Fair Value Elements [Abstract]    
Property, Plant, and Equipment, Fair Value Disclosure $ 41.5