Investments (Unrealized Loss Position for Investments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 786.2	$ 925.6
Less than 12 Months, Unrealized Losses	(19.7)	(40.4)
12 Months or More, Fair Value	202.3	108.9
12 Months or More, Unrealized Losses	(9.5)	(8.0)
Total, Fair Value	988.5	1,034.5
Total, Unrealized Losses	(29.2)	(48.4)
Current [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	190.8	225.3
Less than 12 Months, Unrealized Losses	(5.5)	(7.9)
12 Months or More, Fair Value	40.1	22.5
12 Months or More, Unrealized Losses	(1.1)	(0.8)
Total, Fair Value	230.9	247.8
Total, Unrealized Losses	(6.6)	(8.7)
Current [Member] | U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		4.0
Less than 12 Months, Unrealized Losses		0
12 Months or More, Fair Value		0
12 Months or More, Unrealized Losses		0
Total, Fair Value		4.0
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	369.4	453.5
Less than 12 Months, Unrealized Losses	(9.6)	(23.5)
12 Months or More, Fair Value	146.4	79.7
12 Months or More, Unrealized Losses	(7.9)	(6.8)
Total, Fair Value	515.8	533.2
Total, Unrealized Losses	(17.5)	(30.3)
Current [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	226.0	242.8
Less than 12 Months, Unrealized Losses	(4.6)	(9.0)
12 Months or More, Fair Value	15.8	6.7
12 Months or More, Unrealized Losses	(0.5)	(0.4)
Total, Fair Value	241.8	249.5
Total, Unrealized Losses	(5.1)	(9.4)
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1.9	25.4
Less than 12 Months, Unrealized Losses	(0.2)	(3.2)
12 Months or More, Fair Value	2.4	34.4
12 Months or More, Unrealized Losses	(0.4)	(4.9)
Total, Fair Value	4.3	59.8
Total, Unrealized Losses	(0.6)	(8.1)
Noncurrent [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	0.5
Less than 12 Months, Unrealized Losses	0	(0.1)
12 Months or More, Fair Value	0.7	0.7
12 Months or More, Unrealized Losses	(0.1)	(0.1)
Total, Fair Value	0.7	1.2
Total, Unrealized Losses	(0.1)	(0.2)
Noncurrent [Member] | Obligations of States and Other Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		17.4
Less than 12 Months, Unrealized Losses		(2.2)
12 Months or More, Fair Value		29.6
12 Months or More, Unrealized Losses		(4.1)
Total, Fair Value		47.0
Total, Unrealized Losses		(6.3)
Noncurrent [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1.9	7.5
Less than 12 Months, Unrealized Losses	(0.2)	(0.9)
12 Months or More, Fair Value	1.7	4.1
12 Months or More, Unrealized Losses	(0.3)	(0.7)
Total, Fair Value	3.6	11.6
Total, Unrealized Losses	$ (0.5)	$ (1.6)