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Fair Value (Fair Value Of Financial Assets And Liabilities Measured On A Recurring Basis) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Risk management assets $ 13.2 $ 4.4
Available-for-sale securities 170.9 154.2
Total 184.1 158.6
Risk management liabilities 222.8 146.6
Total 222.8 146.6
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Risk management assets 0.0 0.0
Available-for-sale securities 0.0 0.0
Total 0.0 0.0
Risk management liabilities 0.0 0.0
Total 0.0 0.0
Significant Other Observable Inputs (Level 2) [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Risk management assets 13.2 4.4
Available-for-sale securities 170.9 154.2
Total 184.1 158.6
Risk management liabilities 222.8 146.6
Total 222.8 146.6
Significant Unobservable Inputs (Level 3) [Member]    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Risk management assets 0.0 0.0
Available-for-sale securities 0.0 0.0
Total 0.0 0.0
Risk management liabilities 0.0 0.0
Total $ 0.0 $ 0.0