Fair Value (Fair Value Of Financial Assets And Liabilities Measured On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	$ 24.0	$ 21.1
Available-for-sale securities	138.3	133.9
Total	162.3	155.0
Risk management liabilities	51.7	71.7
Total	51.7	71.7
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	24.0	21.1
Available-for-sale securities	138.3	133.9
Total	162.3	155.0
Risk management liabilities	51.7	71.4
Total	51.7	71.4
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	0.3
Total	$ 0.0	$ 0.3