Investments (Unrealized Loss Position for Investments) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 1,119.6	$ 303.0
Less than 12 Months, Unrealized Losses	(49.3)	(2.7)
12 Months or More, Fair Value	1.9	0.3
12 Months or More, Unrealized Losses	(0.1)	0
Total, Fair Value	1,121.5	303.3
Total, Unrealized Losses	(49.4)	(2.7)
Current [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	254.4	54.9
Less than 12 Months, Unrealized Losses	(7.5)	(0.2)
12 Months or More, Fair Value	0.4	0.1
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	254.8	55.0
Total, Unrealized Losses	(7.5)	(0.2)
Current [Member] | U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	3.2	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	3.2	10.1
Total, Unrealized Losses	0	0
Current [Member] | Obligations of States and Other Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	578.9	192.1
Less than 12 Months, Unrealized Losses	(30.6)	(2.0)
12 Months or More, Fair Value	0.2	0.2
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	579.1	192.3
Total, Unrealized Losses	(30.6)	(2.0)
Current [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	283.1	45.9
Less than 12 Months, Unrealized Losses	(11.2)	(0.5)
12 Months or More, Fair Value	1.3	0
12 Months or More, Unrealized Losses	(0.1)	0
Total, Fair Value	284.4	45.9
Total, Unrealized Losses	(11.3)	(0.5)
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	17.5
Less than 12 Months, Unrealized Losses	(2.3)
12 Months or More, Fair Value	0.8
12 Months or More, Unrealized Losses	(0.1)
Total, Fair Value	18.3
Total, Unrealized Losses	(2.4)
Noncurrent [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1.1
Less than 12 Months, Unrealized Losses	(0.1)
12 Months or More, Fair Value	0
12 Months or More, Unrealized Losses	0
Total, Fair Value	1.1
Total, Unrealized Losses	(0.1)
Noncurrent [Member] | Obligations of States and Other Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	12.6
Less than 12 Months, Unrealized Losses	(1.6)
12 Months or More, Fair Value	0
12 Months or More, Unrealized Losses	0
Total, Fair Value	12.6
Total, Unrealized Losses	(1.6)
Noncurrent [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	3.8
Less than 12 Months, Unrealized Losses	(0.6)
12 Months or More, Fair Value	0.8
12 Months or More, Unrealized Losses	(0.1)
Total, Fair Value	4.6
Total, Unrealized Losses	$ (0.7)