Restricted Securities Available For Sale (Restricted Securities Available For Sale) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cost		$ 46.4	$ 45.5
Gross Unrealized Gains		0.0	0.1
Gross Unrealized Losses		0.3	0.3
Total restricted securities available for sale	[1]	46.1	45.3
US Government And Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost		19.2	20.4
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.2	0.1
Total restricted securities available for sale		19.0	20.3
Corporate bonds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost		18.2	16.9
Gross Unrealized Gains		0.0	0.1
Gross Unrealized Losses		0.1	0.2
Total restricted securities available for sale		18.1	16.8
Asset-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost		6.6	5.0
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0	0.0
Total restricted securities available for sale		6.6	5.0
Mortgage-backed securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost		2.4	3.2
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0	0.0
Total restricted securities available for sale		$ 2.4	$ 3.2

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