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Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis $ 62.4 $ 58.3
Gross Unrealized Gains 1.2 1.0
Gross Unrealized Losses (0.4) 0.0
Estimated Fair Value [1] 63.2 59.3
Corporate Bond Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 25.3 25.3
Gross Unrealized Gains 0.3 0.5
Gross Unrealized Losses (0.3) 0.0
Estimated Fair Value 25.3 25.8
US Government and Agency Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 22.8 21.3
Gross Unrealized Gains 0.8 0.4
Gross Unrealized Losses 0.0 0.0
Estimated Fair Value 23.6 21.7
Asset-backed Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 10.8 11.2
Gross Unrealized Gains 0.1 0.1
Gross Unrealized Losses (0.1)  
Estimated Fair Value 10.8 11.3
Commercial Paper [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 3.0  
Gross Unrealized Gains 0.0  
Gross Unrealized Losses 0.0  
Estimated Fair Value 3.0  
Mortgage-backed Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 0.5 0.5
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Estimated Fair Value $ 0.5 $ 0.5
[1]
Measured at fair value on a recurring basis.