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Fair Value (Available-For-Sale Securities) (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2023
Dec. 31, 2022
Fair Value Disclosure [Line Items]    
Amortized Cost $ 178.2 $ 166.7
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (15.4) (14.2)
Allowance for Credit Loss (0.9) (0.9)
Fair Value 161.9 151.6
Debt Securities, Available-for-Sale, Realized Gain 0.6  
U.S. Treasury debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 69.0 67.7
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (5.0) (4.5)
Allowance for Credit Loss 0.0 0.0
Fair Value 64.0 63.2
Debt Securities, Available-for-sale, Unrealized Loss Position 59.2 61.0
Corporate/Other debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 109.2 99.0
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (10.4) (9.7)
Allowance for Credit Loss (0.9) (0.9)
Fair Value 97.9 88.4
Debt Securities, Available-for-sale, Unrealized Loss Position $ 92.3 $ 85.5