Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	$ 51.6	$ 53.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.2)	(0.3)
Estimated Fair Value	51.4	53.6
US Government and Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	31.6	22.3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.1)	(0.2)
Estimated Fair Value	31.5	22.1
Corporate Bonds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	16.5	3.1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.1)	(0.1)
Estimated Fair Value	16.4	3.0
Asset-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	2.8
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	2.8
Commercial Paper [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost		23.9
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		23.9
Certificates of Deposit [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	0.7	4.6
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 0.7	$ 4.6