Investments (Unrealized Loss Position for Investments) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 1,028.5	$ 303.0
Less than 12 Months, Unrealized Losses	(44.1)	(2.7)
12 Months or More, Fair Value	11.8	0.3
12 Months or More, Unrealized Losses	(0.6)	0
Total, Fair Value	1,040.3	303.3
Total, Unrealized Losses	(44.7)	(2.7)
Current [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	232.8	54.9
Less than 12 Months, Unrealized Losses	(6.2)	(0.2)
12 Months or More, Fair Value	9.1	0.1
12 Months or More, Unrealized Losses	(0.4)	0
Total, Fair Value	241.9	55.0
Total, Unrealized Losses	(6.6)	(0.2)
Current [Member] | U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		10.1
Less than 12 Months, Unrealized Losses		0
12 Months or More, Fair Value		0
12 Months or More, Unrealized Losses		0
Total, Fair Value		10.1
Total, Unrealized Losses		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	542.4	192.1
Less than 12 Months, Unrealized Losses	(28.3)	(2.0)
12 Months or More, Fair Value	0.2	0.2
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	542.6	192.3
Total, Unrealized Losses	(28.3)	(2.0)
Current [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	253.3	45.9
Less than 12 Months, Unrealized Losses	(9.6)	(0.5)
12 Months or More, Fair Value	2.5	0
12 Months or More, Unrealized Losses	(0.2)	0
Total, Fair Value	255.8	45.9
Total, Unrealized Losses	(9.8)	(0.5)
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	51.9
Less than 12 Months, Unrealized Losses	(7.4)
12 Months or More, Fair Value	0.7
12 Months or More, Unrealized Losses	(0.1)
Total, Fair Value	52.6
Total, Unrealized Losses	(7.5)
Noncurrent [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0
Less than 12 Months, Unrealized Losses	0
12 Months or More, Fair Value	0.7
12 Months or More, Unrealized Losses	(0.1)
Total, Fair Value	0.7
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	42.9
Less than 12 Months, Unrealized Losses	(6.0)
12 Months or More, Fair Value	0
12 Months or More, Unrealized Losses	0
Total, Fair Value	42.9
Total, Unrealized Losses	(6.0)
Noncurrent [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	9.0
Less than 12 Months, Unrealized Losses	(1.4)
12 Months or More, Fair Value	0
12 Months or More, Unrealized Losses	0
Total, Fair Value	9.0
Total, Unrealized Losses	$ (1.4)