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Fair Value (Available-For-Sale Securities) (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Fair Value Disclosure [Line Items]    
Amortized Cost $ 169.0 $ 166.7
Gross Unrealized Gains 0.8 0.0
Gross Unrealized Losses 10.1 14.2
Allowance for Credit Loss (0.6) 0.9
Fair Value 159.1 151.6
U.S. Treasury debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 63.8 67.7
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 3.2 4.5
Allowance for Credit Loss 0.0 0.0
Fair Value 60.6 63.2
Debt Securities, Available-for-sale, Unrealized Loss Position 58.7 61.0
Corporate/Other debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 105.2 99.0
Gross Unrealized Gains 0.8 0.0
Gross Unrealized Losses 6.9 9.7
Allowance for Credit Loss (0.6) 0.9
Fair Value 98.5 88.4
Debt Securities, Available-for-sale, Unrealized Loss Position $ 74.8 $ 85.5