Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale by Aging Catagory) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	$ 36.0	$ 28.2
Gross Unrealized Losses, Less than 12 Months	(0.5)	(0.2)
Estimated Fair Value, 12 Months or More	10.0	12.0
Gross Unrealized Losses, 12 Months or More	(0.2)	(0.1)
Total Estimated Fair Value	46.0	40.2
Total Gross Unrealized Losses	(0.7)	(0.3)
Corporate Bond Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	18.4	11.1
Gross Unrealized Losses, Less than 12 Months	(0.2)	(0.1)
Estimated Fair Value, 12 Months or More	1.4	1.9
Gross Unrealized Losses, 12 Months or More	(0.1)	0.0
Total Estimated Fair Value	19.8	13.0
Total Gross Unrealized Losses	(0.3)	(0.1)
US Government and Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	10.8	11.0
Gross Unrealized Losses, Less than 12 Months	(0.2)	(0.1)
Estimated Fair Value, 12 Months or More	6.5	7.9
Gross Unrealized Losses, 12 Months or More	(0.1)	(0.1)
Total Estimated Fair Value	17.3	18.9
Total Gross Unrealized Losses	(0.3)	(0.2)
Asset-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	5.7	4.9
Gross Unrealized Losses, Less than 12 Months	(0.1)
Estimated Fair Value, 12 Months or More	1.0	1.0
Gross Unrealized Losses, 12 Months or More	0.0	0.0
Total Estimated Fair Value	6.7	5.9
Total Gross Unrealized Losses	(0.1)
Mortgage-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	1.1	1.2
Estimated Fair Value, 12 Months or More	1.1	1.2
Gross Unrealized Losses, 12 Months or More	0.0	0.0
Total Estimated Fair Value	$ 2.2	$ 2.4