Investments (Unrealized Loss Position for Investments) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 1,280.2	$ 506.4
Less than 12 Months, Unrealized Losses	(16.4)	(4.5)
12 Months or More, Fair Value	83.2	273.3
12 Months or More, Unrealized Losses	(1.8)	(4.4)
Total, Fair Value	1,363.4	779.7
Total, Unrealized Losses	(18.2)	(8.9)
Current [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	360.8	149.3
Less than 12 Months, Unrealized Losses	(3.1)	(0.5)
12 Months or More, Fair Value	40.9	112.5
12 Months or More, Unrealized Losses	(0.5)	(1.4)
Total, Fair Value	401.7	261.8
Total, Unrealized Losses	(3.6)	(1.9)
Current [Member] | U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1.8	20.7
Less than 12 Months, Unrealized Losses	0.0	0.0
12 Months or More, Fair Value	0.0	0.0
12 Months or More, Unrealized Losses	0.0	0.0
Total, Fair Value	1.8	20.7
Total, Unrealized Losses	0.0	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	503.0	37.3
Less than 12 Months, Unrealized Losses	(8.3)	(0.1)
12 Months or More, Fair Value	19.4	104.8
12 Months or More, Unrealized Losses	(0.7)	(1.6)
Total, Fair Value	522.4	142.1
Total, Unrealized Losses	(9.0)	(1.7)
Current [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	414.6	299.1
Less than 12 Months, Unrealized Losses	(5.0)	(3.9)
12 Months or More, Fair Value	22.9	56.0
12 Months or More, Unrealized Losses	(0.6)	(1.4)
Total, Fair Value	437.5	355.1
Total, Unrealized Losses	(5.6)	(5.3)
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	4.4	4.0
Less than 12 Months, Unrealized Losses	(0.7)	(0.7)
12 Months or More, Fair Value	2.1	0.6
12 Months or More, Unrealized Losses	(0.4)	(0.2)
Total, Fair Value	6.5	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	0.0
Less than 12 Months, Unrealized Losses	0.0	0.0
12 Months or More, Fair Value	0.4	0.6
12 Months or More, Unrealized Losses	(0.2)	(0.2)
Total, Fair Value	0.4	0.6
Total, Unrealized Losses	(0.2)	(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	0.0
Less than 12 Months, Unrealized Losses	0.0
12 Months or More, Fair Value	1.5
12 Months or More, Unrealized Losses	(0.2)
Total, Fair Value	1.5
Total, Unrealized Losses	(0.2)
Noncurrent [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	4.4	4.0
Less than 12 Months, Unrealized Losses	(0.7)	(0.7)
12 Months or More, Fair Value	0.2	0.0
12 Months or More, Unrealized Losses	0.0	0.0
Total, Fair Value	4.6	4.0
Total, Unrealized Losses	$ (0.7)	$ (0.7)