Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Restricted Securities Available For Sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 57.5	$ 46.4
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.8)	(0.3)
Estimated Fair Value	[1]	56.7	46.1
Corporate Bond Securities [Member]
Restricted Securities Available For Sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		23.5	18.2
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.3)	(0.1)
Estimated Fair Value		23.2	18.1
US Government and Agency Securities [Member]
Restricted Securities Available For Sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		22.0	19.2
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.4)	(0.2)
Estimated Fair Value		21.6	19.0
Asset-backed Securities [Member]
Restricted Securities Available For Sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		9.7	6.6
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.1)
Estimated Fair Value		9.6	6.6
Mortgage-backed Securities [Member]
Restricted Securities Available For Sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		1.4	2.4
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0	0.0
Estimated Fair Value		1.4	$ 2.4
Commercial Paper [Member]
Restricted Securities Available For Sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		0.9
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		$ 0.9

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