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Investments - Contractual Maturity (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Amortized Cost [Abstract]    
Amortized Cost Due within 1 year $ 9.2  
Amortized Cost After 1 year but within 5 years 28.8  
Amortized Cost After 5 years but within 10 years 13.4  
Amortized Cost Due after 10 years 2.6  
Amortized Cost 54.0  
Fair Value [Abstract]    
Fair Value Due within 1 year 9.2  
Fair Value After 1 year but within 5 years 26.0  
Fair Value After 5 years but within 10 years 13.4  
Fair Value Due after 10 years 2.6  
Available-for-sale securities, fair value $ 51.2 $ 58.4