Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 46.6	$ 48.5
Gross Unrealized Gains	0.4	0.0
Gross Unrealized Losses	0.0	(0.2)
Estimated Fair Value	47.0	48.3
US Government and Agency Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis	19.6	21.8
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	19.8	21.7
Corporate Bond Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis	17.2	17.1
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	17.4	17.0
Asset-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis	5.5	5.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0
Estimated Fair Value	5.5	5.5
Mortgage-backed Securities [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis	3.6	4.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	3.6	$ 4.1
Commercial Paper [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Available-for-sale Securities, Amortized Cost Basis	0.7
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Estimated Fair Value	$ 0.7