Investments (Unrealized Loss Position for Investments) (Details) (Current [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 523.9	$ 303.0
Less than 12 Months, Unrealized Losses	(6.6)	(2.7)
12 Months or More, Fair Value	2.3	0.3
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	526.2	303.3
Total, Unrealized Losses	(6.6)	(2.7)
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	122.2	54.9
Less than 12 Months, Unrealized Losses	(0.7)	(0.2)
12 Months or More, Fair Value	0.7	0.1
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	122.9	55.0
Total, Unrealized Losses	(0.7)	(0.2)
U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	8.0	10.1
Less than 12 Months, Unrealized Losses	0	0
12 Months or More, Fair Value	0	0
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	8.0	10.1
Total, Unrealized Losses	0	0
Obligations of States and Other Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	321.6	192.1
Less than 12 Months, Unrealized Losses	(5.1)	(2.0)
12 Months or More, Fair Value	0.2	0.2
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	321.8	192.3
Total, Unrealized Losses	(5.1)	(2.0)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	72.1	45.9
Less than 12 Months, Unrealized Losses	(0.8)	(0.5)
12 Months or More, Fair Value	1.4	0
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	73.5	45.9
Total, Unrealized Losses	$ (0.8)	$ (0.5)