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Fair Value (Available-For-Sale Securities) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Fair Value Disclosure [Line Items]    
Amortized Cost $ 163.9 $ 150.1
Gross Unrealized Gains 8.0 4.3
Gross Unrealized Losses 0.5 0.2
Allowance for Credit Loss (0.5) 0.0
Fair Value 170.9 154.2
Debt Securities, Available-for-sale, Unrealized Loss Position 0.0 17.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 13.2 12.2
U.S. Treasury debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 33.7 31.4
Gross Unrealized Gains 0.3 0.1
Gross Unrealized Losses 0.0 0.1
Allowance for Credit Loss 0.0 0.0
Fair Value 34.0 31.4
Corporate/Other debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 130.2 118.7
Gross Unrealized Gains 7.7 4.2
Gross Unrealized Losses 0.5 0.1
Allowance for Credit Loss (0.5) 0.0
Fair Value $ 136.9 $ 122.8