Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	$ 54.7	$ 53.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.1)	(0.3)
Estimated Fair Value	54.6	53.6
US Government and Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	30.6	22.3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.1)	(0.2)
Estimated Fair Value	30.5	22.1
Corporate Bonds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	11.6	3.1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(0.1)
Estimated Fair Value	11.6	3.0
Commercial Paper [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	8.4	23.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	8.4	23.9
Certificates of Deposit [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	4.1	4.6
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 4.1	$ 4.6