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Fair Value (Available-For-Sale Securities) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Fair Value Disclosure [Line Items]    
Amortized Cost $ 141.3 $ 133.7
Gross Unrealized Gains 0.5 0.9
Gross Unrealized Losses 3.5 0.7
Fair Value 138.3 133.9
U.S. Treasury debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 23.6 26.9
Gross Unrealized Gains 0.1 0.0
Gross Unrealized Losses 0.1 0.1
Fair Value 23.6 26.8
Corporate/Other debt securities    
Fair Value Disclosure [Line Items]    
Amortized Cost 117.7 106.8
Gross Unrealized Gains 0.4 0.9
Gross Unrealized Losses 3.4 0.6
Fair Value $ 114.7 $ 107.1