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Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis $ 55.2 $ 46.4
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.8) (0.3)
Estimated Fair Value [1] 54.4 46.1
Corporate Bond Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 22.9 18.2
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.4) (0.1)
Estimated Fair Value 22.5 18.1
US Government and Agency Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 20.9 19.2
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.3) (0.2)
Estimated Fair Value 20.6 19.0
Asset-backed Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 9.4 6.6
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.1)  
Estimated Fair Value 9.3 6.6
Mortgage-backed Securities [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 1.5 2.4
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Estimated Fair Value 1.5 $ 2.4
Commercial Paper [Member]    
Restricted Securities Available For Sale [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 0.5  
Gross Unrealized Gains 0.0  
Gross Unrealized Losses 0.0  
Estimated Fair Value $ 0.5  
[1] Measured and recorded at fair value on a recurring basis.