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Fair Value (Available-For-Sale Securities) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Fair Value Disclosure [Line Items]          
Amortized Cost $ 143.5   $ 143.5   $ 169.0
Gross Unrealized Gains 1.1   1.1   0.8
Gross Unrealized Losses (6.5)   (6.5)   (10.1)
Allowance for Credit Loss (0.2)   (0.2)   (0.6)
Fair Value 137.9   137.9   159.1
Debt Securities, Available-for-Sale, Realized Gain 0.1 $ 0.0 0.5 $ 0.6  
U.S. Treasury debt securities          
Fair Value Disclosure [Line Items]          
Amortized Cost 49.1   49.1   63.8
Gross Unrealized Gains 0.0   0.0   0.0
Gross Unrealized Losses (1.9)   (1.9)   (3.2)
Allowance for Credit Loss 0.0   0.0   0.0
Fair Value 47.2   47.2   60.6
Debt Securities, Available-for-sale, Unrealized Loss Position 41.2   41.2   58.7
Corporate/Other debt securities          
Fair Value Disclosure [Line Items]          
Amortized Cost 94.4   94.4   105.2
Gross Unrealized Gains 1.1   1.1   0.8
Gross Unrealized Losses (4.6)   (4.6)   (6.9)
Allowance for Credit Loss (0.2)   (0.2)   (0.6)
Fair Value 90.7   90.7   98.5
Debt Securities, Available-for-sale, Unrealized Loss Position $ 66.5   $ 66.5   $ 74.8