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Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale by Aging Catagory) (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Restricted Securities Available For Sale [Line Items]    
Estimated Fair Value, Less than 12 Months $ 18.8 $ 5.2
Gross Unrealized Losses, Less than 12 Months (0.4) 0.0
Estimated Fair Value, 12 Months or More 0.0 0.0
Gross Unrealized Losses, 12 Months or More 0.0 0.0
Total Estimated Fair Value 18.8 5.2
Total Gross Unrealized Losses (0.4) 0.0
Corporate Bond Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Estimated Fair Value, Less than 12 Months 12.2 1.4
Gross Unrealized Losses, Less than 12 Months (0.3) 0.0
Estimated Fair Value, 12 Months or More 0.0 0.0
Gross Unrealized Losses, 12 Months or More 0.0 0.0
Total Estimated Fair Value 12.2 1.4
Total Gross Unrealized Losses (0.3) 0.0
US Government and Agency Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Estimated Fair Value, Less than 12 Months 0.0 1.9
Gross Unrealized Losses, Less than 12 Months 0.0 0.0
Estimated Fair Value, 12 Months or More 0.0 0.0
Gross Unrealized Losses, 12 Months or More 0.0 0.0
Total Estimated Fair Value 0.0 1.9
Total Gross Unrealized Losses 0.0 0.0
Asset-backed Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Estimated Fair Value, Less than 12 Months 6.3 1.9
Gross Unrealized Losses, Less than 12 Months (0.1)  
Estimated Fair Value, 12 Months or More 0.0 0.0
Gross Unrealized Losses, 12 Months or More 0.0 0.0
Total Estimated Fair Value 6.3 1.9
Total Gross Unrealized Losses (0.1)  
Commercial Paper [Member]    
Restricted Securities Available For Sale [Line Items]    
Estimated Fair Value, Less than 12 Months 0.0  
Estimated Fair Value, 12 Months or More 0.0  
Gross Unrealized Losses, 12 Months or More 0.0  
Total Estimated Fair Value 0.0  
Mortgage-backed Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Estimated Fair Value, Less than 12 Months 0.3 0.0
Estimated Fair Value, 12 Months or More 0.0 0.0
Gross Unrealized Losses, 12 Months or More 0.0 0.0
Total Estimated Fair Value $ 0.3 $ 0.0