Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 46.1	$ 45.5
Gross Unrealized Gains		0.1	0.1
Gross Unrealized Losses		(0.1)	(0.3)
Estimated Fair Value	[1]	46.1	45.3
US Government and Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		18.7	20.4
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		(0.1)	(0.1)
Estimated Fair Value		18.6	20.3
Corporate Bond Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		18.2	16.9
Gross Unrealized Gains		0.1	0.1
Gross Unrealized Losses		0.0	(0.2)
Estimated Fair Value		18.3	16.8
Asset-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		6.9	5.0
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		6.9	5.0
Mortgage-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis		2.3	3.2
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0	0.0
Estimated Fair Value		$ 2.3	$ 3.2

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