Investments (Unrealized Loss Position for Investments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 541.5	$ 506.4
Less than 12 Months, Unrealized Losses	(3.1)	(4.5)
12 Months or More, Fair Value	101.1	273.3
12 Months or More, Unrealized Losses	(1.7)	(4.4)
Total, Fair Value	642.6	779.7
Total, Unrealized Losses	(4.8)	(8.9)
Current [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	205.4	149.3
Less than 12 Months, Unrealized Losses	(0.6)	(0.5)
12 Months or More, Fair Value	45.5	112.5
12 Months or More, Unrealized Losses	(0.4)	(1.4)
Total, Fair Value	250.9	261.8
Total, Unrealized Losses	(1.0)	(1.9)
Current [Member] | U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	23.7	20.7
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	23.7	20.7
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	203.6	37.3
Less than 12 Months, Unrealized Losses	(1.4)	(0.1)
12 Months or More, Fair Value	28.9	104.8
12 Months or More, Unrealized Losses	(0.9)	(1.6)
Total, Fair Value	232.5	142.1
Total, Unrealized Losses	(2.3)	(1.7)
Current [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	108.8	299.1
Less than 12 Months, Unrealized Losses	(1.1)	(3.9)
12 Months or More, Fair Value	26.7	56.0
12 Months or More, Unrealized Losses	(0.4)	(1.4)
Total, Fair Value	135.5	355.1
Total, Unrealized Losses	(1.5)	(5.3)
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5.7	4.0
Less than 12 Months, Unrealized Losses	(0.9)	(0.7)
12 Months or More, Fair Value	0.6	0.6
12 Months or More, Unrealized Losses	(0.2)	(0.2)
Total, Fair Value	6.3	4.6
Total, Unrealized Losses	(1.1)	(0.9)
Noncurrent [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	0
Less than 12 Months, Unrealized Losses	0	0
12 Months or More, Fair Value	0.6	0.6
12 Months or More, Unrealized Losses	(0.2)	(0.2)
Total, Fair Value	0.6	0.6
Total, Unrealized Losses	(0.2)	(0.2)
Noncurrent [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5.7	4.0
Less than 12 Months, Unrealized Losses	(0.9)	(0.7)
12 Months or More, Fair Value	0	0
12 Months or More, Unrealized Losses	0	0
Total, Fair Value	5.7	4.0
Total, Unrealized Losses	$ (0.9)	$ (0.7)