Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 52.0	$ 46.4
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.7)	(0.3)
Estimated Fair Value	[1]	51.3	46.1
US Government and Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		21.3	19.2
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.3)	(0.2)
Estimated Fair Value		21.0	19.0
Corporate Bond Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		20.5	18.2
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.3)	(0.1)
Estimated Fair Value		20.2	18.1
Asset-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		8.0	6.6
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.1)
Estimated Fair Value		7.9	6.6
Mortgage-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		2.2	2.4
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0	0.0
Estimated Fair Value		$ 2.2	$ 2.4

[1] Measured and recorded at fair value on a recurring basis.