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Fair Value (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Fair Value Disclosure [Line Items]    
Transfers between Fair Value Hierarchies $ 0.0 $ 0.0
Material Level 3 Changes 0.0 $ 0.0
U.S. Treasury debt securities    
Fair Value Disclosure [Line Items]    
Available-for-sale Securities, Maturities, Next Twelve Months, Fair Value 4.7  
Corporate/Other debt securities    
Fair Value Disclosure [Line Items]    
Available-for-sale Securities, Maturities, Next Twelve Months, Fair Value $ 6.3