Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale by Aging Catagory) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	$ 34.5	$ 26.0
Gross Unrealized Losses, Less than 12 Months	(0.1)	(0.3)
Estimated Fair Value, 12 Months or More	7.0	0
Gross Unrealized Losses, 12 Months or More	(0.1)	0
Total Estimated Fair Value	41.5	26.0
Total Gross Unrealized Losses	(0.2)	(0.3)
US Government and Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	20.8	11.8
Gross Unrealized Losses, Less than 12 Months	(0.1)	(0.2)
Estimated Fair Value, 12 Months or More	4.3	0
Gross Unrealized Losses, 12 Months or More	0	0
Total Estimated Fair Value	25.1	11.8
Total Gross Unrealized Losses	(0.1)	(0.2)
Corporate Bonds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	12.1	2.3
Gross Unrealized Losses, Less than 12 Months		(0.1)
Estimated Fair Value, 12 Months or More	2.7	0
Gross Unrealized Losses, 12 Months or More	(0.1)	0
Total Estimated Fair Value	14.8	2.3
Total Gross Unrealized Losses	(0.1)	(0.1)
Asset-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months	1.6
Gross Unrealized Losses, Less than 12 Months
Estimated Fair Value, 12 Months or More	0
Gross Unrealized Losses, 12 Months or More	0
Total Estimated Fair Value	1.6
Total Gross Unrealized Losses
Commercial Paper [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months		10.0
Gross Unrealized Losses, Less than 12 Months		0
Estimated Fair Value, 12 Months or More		0
Gross Unrealized Losses, 12 Months or More		0
Total Estimated Fair Value		10.0
Total Gross Unrealized Losses		0
Certificates of Deposit [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Estimated Fair Value, Less than 12 Months		1.9
Gross Unrealized Losses, Less than 12 Months
Estimated Fair Value, 12 Months or More		0
Gross Unrealized Losses, 12 Months or More		0
Total Estimated Fair Value		1.9
Total Gross Unrealized Losses