Investments (Unrealized Loss Position for Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 865.1	$ 506.4
Less than 12 Months, Unrealized Losses	(9.0)	(4.5)
12 Months or More, Fair Value	68.4	273.3
12 Months or More, Unrealized Losses	(1.9)	(4.4)
Total, Fair Value	933.5	779.7
Total, Unrealized Losses	(10.9)	(8.9)
Current [Member] | Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	262.2	149.3
Less than 12 Months, Unrealized Losses	(1.6)	(0.5)
12 Months or More, Fair Value	23.9	112.5
12 Months or More, Unrealized Losses	(0.2)	(1.4)
Total, Fair Value	286.1	261.8
Total, Unrealized Losses	(1.8)	(1.9)
Current [Member] | U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	12.3	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	12.3	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	211.2	37.3
Less than 12 Months, Unrealized Losses	(2.3)	(0.1)
12 Months or More, Fair Value	17.2	104.8
12 Months or More, Unrealized Losses	(0.7)	(1.6)
Total, Fair Value	228.4	142.1
Total, Unrealized Losses	(3.0)	(1.7)
Current [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	379.4	299.1
Less than 12 Months, Unrealized Losses	(5.1)	(3.9)
12 Months or More, Fair Value	27.3	56.0
12 Months or More, Unrealized Losses	(1.0)	(1.4)
Total, Fair Value	406.7	355.1
Total, Unrealized Losses	(6.1)	(5.3)
Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	11.1	4.0
Less than 12 Months, Unrealized Losses	(1.9)	(0.7)
12 Months or More, Fair Value	9.0	0.6
12 Months or More, Unrealized Losses	(1.8)	(0.2)
Total, Fair Value	20.1	4.6
Total, Unrealized Losses	(3.7)	(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	(0.1)	(0.2)
Total, Fair Value	0.4	0.6
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	0.0
Less than 12 Months, Unrealized Losses	0.0
12 Months or More, Fair Value	1.5
12 Months or More, Unrealized Losses	(0.1)
Total, Fair Value	1.5
Total, Unrealized Losses	(0.1)
Noncurrent [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	11.1	4.0
Less than 12 Months, Unrealized Losses	(1.9)	(0.7)
12 Months or More, Fair Value	7.1	0.0
12 Months or More, Unrealized Losses	(1.6)	0.0
Total, Fair Value	18.2	4.0
Total, Unrealized Losses	$ (3.5)	$ (0.7)