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Fair Value (Available-For-Sale Securities) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Dec. 31, 2023
Fair Value Disclosure [Line Items]    
Amortized Cost $ 160.2 $ 169.0
Gross Unrealized Gains 0.6 0.8
Gross Unrealized Losses (10.3) (10.1)
Allowance for Credit Loss (0.4) (0.6)
Fair Value 150.1 159.1
Debt Securities, Available-for-Sale, Realized Gain 0.4  
U.S. Treasury debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 62.1 63.8
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (3.3) (3.2)
Allowance for Credit Loss 0.0 0.0
Fair Value 58.8 60.6
Debt Securities, Available-for-sale, Unrealized Loss Position 58.8 58.7
Corporate/Other debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 98.1 105.2
Gross Unrealized Gains 0.6 0.8
Gross Unrealized Losses (7.0) (6.9)
Allowance for Credit Loss (0.4) (0.6)
Fair Value 91.3 98.5
Debt Securities, Available-for-sale, Unrealized Loss Position $ 72.7 $ 74.8