Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 101.5	$ 13.9
12 months or less, Fair Value	4,229.1	1,885.3
Greater than 12 months, Gross Unrealized Losses	68.8	31.9
Greater than 12 months, Fair Value	1,172.6	1,168.4
Total, Gross Unrealized Losses	170.3	45.8
Total, Fair Value	5,401.7	3,053.7
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.6
12 months or less, Fair Value	151.9
Greater than 12 months, Gross Unrealized Losses	6.4
Greater than 12 months, Fair Value	38.1
Total, Gross Unrealized Losses	11.0
Total, Fair Value	190.0
US Treasury and Government Agencies [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.0	2.1
12 months or less, Fair Value	393.6	282.1
Greater than 12 months, Gross Unrealized Losses	6.3	3.5
Greater than 12 months, Fair Value	103.2	103.1
Total, Gross Unrealized Losses	16.3	5.6
Total, Fair Value	496.8	385.2
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.3	0.9
12 months or less, Fair Value	86.6	99.4
Greater than 12 months, Gross Unrealized Losses	0.9	0.4
Greater than 12 months, Fair Value	52.5	22.8
Total, Gross Unrealized Losses	2.2	1.3
Total, Fair Value	139.1	122.2
Municipal [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	8.5	2.1
12 months or less, Fair Value	390.7	257.5
Greater than 12 months, Gross Unrealized Losses	7.0	5.0
Greater than 12 months, Fair Value	126.6	133.1
Total, Gross Unrealized Losses	15.5	7.1
Total, Fair Value	517.3	390.6
Municipal [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	0.9
Total, Fair Value	0.9
Corporate [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	57.2	5.6
12 months or less, Fair Value	2,343.5	799.6
Greater than 12 months, Gross Unrealized Losses	33.1	12.3
Greater than 12 months, Fair Value	480.1	481.3
Total, Gross Unrealized Losses	90.3	17.9
Total, Fair Value	2,823.6	1,280.9
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.6	1.7
12 months or less, Fair Value	151.0	53.2
Greater than 12 months, Gross Unrealized Losses	6.4	6.8
Greater than 12 months, Fair Value	38.1	43.5
Total, Gross Unrealized Losses	11.0	8.5
Total, Fair Value	189.1	96.7
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	13.1	2.0
12 months or less, Fair Value	512.5	272.9
Greater than 12 months, Gross Unrealized Losses	18.4	9.1
Greater than 12 months, Fair Value	347.4	362.4
Total, Gross Unrealized Losses	31.5	11.1
Total, Fair Value	859.9	635.3
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.4	0.9
12 months or less, Fair Value	445.6	139.3
Greater than 12 months, Gross Unrealized Losses	3.0	1.6
Greater than 12 months, Fair Value	61.2	63.2
Total, Gross Unrealized Losses	13.4	2.5
Total, Fair Value	506.8	202.5
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.0	0.3
12 months or less, Fair Value	56.6	34.5
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	1.6	2.5
Total, Gross Unrealized Losses	1.1	0.3
Total, Fair Value	58.2	37.0
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	106.1	15.6
12 months or less, Fair Value	4,381.0	1,938.5
Greater than 12 months, Gross Unrealized Losses	75.2	38.7
Greater than 12 months, Fair Value	1,210.7	1,211.9
Total, Gross Unrealized Losses	181.3	54.3
Total, Fair Value	$ 5,591.7	$ 3,150.4