Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 8.9	$ 70.5
12 months or less, Fair Value	270.0	2,501.1
Greater than 12 months, Gross Unrealized Losses	24.0	52.0
Greater than 12 months, Fair Value	295.4	386.6
Total, Gross Unrealized Losses	32.9	122.5
Total, Fair Value	565.4	2,887.7
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.0	43.3
12 months or less, Fair Value	112.5	2,168.8
Greater than 12 months, Gross Unrealized Losses	7.8	25.5
Greater than 12 months, Fair Value	197.3	323.4
Total, Gross Unrealized Losses	9.8	68.8
Total, Fair Value	309.8	2,492.2
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	7.0	62.9
12 months or less, Fair Value	193.8	2,334.3
Greater than 12 months, Gross Unrealized Losses	24.0	52.0
Greater than 12 months, Fair Value	295.4	386.6
Total, Gross Unrealized Losses	31.0	114.9
Total, Fair Value	489.2	2,720.9
US Treasury and Government Agencies [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		1.5
12 months or less, Fair Value	5.0	139.0
Greater than 12 months, Gross Unrealized Losses		2.0
Greater than 12 months, Fair Value		77.2
Total, Gross Unrealized Losses		3.5
Total, Fair Value	5.0	216.2
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.8
12 months or less, Fair Value	4.1	63.6
Greater than 12 months, Gross Unrealized Losses		0.7
Greater than 12 months, Fair Value	1.2	8.4
Total, Gross Unrealized Losses		1.5
Total, Fair Value	5.3	72.0
Municipal [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	2.3
12 months or less, Fair Value	12.8	143.0
Greater than 12 months, Gross Unrealized Losses	0.7	1.9
Greater than 12 months, Fair Value	33.0	57.4
Total, Gross Unrealized Losses	0.9	4.2
Total, Fair Value	45.8	200.4
Corporate [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.4	30.7
12 months or less, Fair Value	51.6	1,138.3
Greater than 12 months, Gross Unrealized Losses	6.2	18.9
Greater than 12 months, Fair Value	113.9	122.3
Total, Gross Unrealized Losses	7.6	49.6
Total, Fair Value	165.5	1,260.6
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.0	19.6
12 months or less, Fair Value	81.3	165.5
Greater than 12 months, Gross Unrealized Losses	16.2	26.5
Greater than 12 months, Fair Value	98.1	63.2
Total, Gross Unrealized Losses	21.2	46.1
Total, Fair Value	179.4	228.7
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	3.0
12 months or less, Fair Value	25.1	334.5
Greater than 12 months, Gross Unrealized Losses	0.7	1.9
Greater than 12 months, Fair Value	35.6	47.0
Total, Gross Unrealized Losses	0.9	4.9
Total, Fair Value	60.7	381.5
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		4.2
12 months or less, Fair Value	8.1	293.8
Greater than 12 months, Gross Unrealized Losses	0.1	0.1
Greater than 12 months, Fair Value	9.3	9.7
Total, Gross Unrealized Losses	0.1	4.3
Total, Fair Value	17.4	303.5
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	0.8
12 months or less, Fair Value	5.8	56.6
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	4.3	1.4
Total, Gross Unrealized Losses	0.3	0.8
Total, Fair Value	10.1	58.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.9	7.6
12 months or less, Fair Value	76.2	166.8
Total, Gross Unrealized Losses	1.9	7.6
Total, Fair Value	$ 76.2	$ 166.8