Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 4.1	$ 70.5
12 months or less, Fair Value	534.3	2,501.1
Greater than 12 months, Gross Unrealized Losses	19.4	52.0
Greater than 12 months, Fair Value	247.3	386.6
Total, Gross Unrealized Losses	23.5	122.5
Total, Fair Value	781.6	2,887.7
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.3	43.3
12 months or less, Fair Value	485.5	2,168.8
Greater than 12 months, Gross Unrealized Losses	6.2	25.5
Greater than 12 months, Fair Value	142.1	323.4
Total, Gross Unrealized Losses	8.5	68.8
Total, Fair Value	627.6	2,492.2
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.2	62.9
12 months or less, Fair Value	515.6	2,334.3
Greater than 12 months, Gross Unrealized Losses	19.4	52.0
Greater than 12 months, Fair Value	247.3	386.6
Total, Gross Unrealized Losses	22.6	114.9
Total, Fair Value	762.9	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	0.1	1.5
12 months or less, Fair Value	37.8	139.0
Greater than 12 months, Gross Unrealized Losses		2.0
Greater than 12 months, Fair Value		77.2
Total, Gross Unrealized Losses	0.1	3.5
Total, Fair Value	37.8	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	11.5	63.6
Greater than 12 months, Gross Unrealized Losses		0.7
Greater than 12 months, Fair Value	1.5	8.4
Total, Gross Unrealized Losses		1.5
Total, Fair Value	13.0	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	43.7	143.0
Greater than 12 months, Gross Unrealized Losses	0.9	1.9
Greater than 12 months, Fair Value	31.7	57.4
Total, Gross Unrealized Losses	1.1	4.2
Total, Fair Value	75.4	200.4
Corporate [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.3	30.7
12 months or less, Fair Value	209.7	1,138.3
Greater than 12 months, Gross Unrealized Losses	4.3	18.9
Greater than 12 months, Fair Value	55.5	122.3
Total, Gross Unrealized Losses	5.6	49.6
Total, Fair Value	265.2	1,260.6
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.9	19.6
12 months or less, Fair Value	30.1	165.5
Greater than 12 months, Gross Unrealized Losses	13.2	26.5
Greater than 12 months, Fair Value	105.2	63.2
Total, Gross Unrealized Losses	14.1	46.1
Total, Fair Value	135.3	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	122.3	334.5
Greater than 12 months, Gross Unrealized Losses	0.8	1.9
Greater than 12 months, Fair Value	40.1	47.0
Total, Gross Unrealized Losses	1.0	4.9
Total, Fair Value	162.4	381.5
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	4.2
12 months or less, Fair Value	56.8	293.8
Greater than 12 months, Gross Unrealized Losses		0.1
Greater than 12 months, Fair Value	7.1	9.7
Total, Gross Unrealized Losses	0.4	4.3
Total, Fair Value	63.9	303.5
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	0.8
12 months or less, Fair Value	3.7	56.6
Greater than 12 months, Gross Unrealized Losses	0.2
Greater than 12 months, Fair Value	6.2	1.4
Total, Gross Unrealized Losses	0.3	0.8
Total, Fair Value	9.9	58.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.9	7.6
12 months or less, Fair Value	18.7	166.8
Total, Gross Unrealized Losses	0.9	7.6
Total, Fair Value	$ 18.7	$ 166.8