Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 73.6	$ 13.9
12 months or less, Fair Value	3,916.6	1,885.3
Greater than 12 months, Gross Unrealized Losses	58.5	31.9
Greater than 12 months, Fair Value	1,161.0	1,168.4
Total, Gross Unrealized Losses	132.1	45.8
Total, Fair Value	5,077.6	3,053.7
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.7
12 months or less, Fair Value	98.7
Greater than 12 months, Gross Unrealized Losses	6.4
Greater than 12 months, Fair Value	39.0
Total, Gross Unrealized Losses	10.1
Total, Fair Value	137.7
US Treasury and Government Agencies [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	7.1	2.1
12 months or less, Fair Value	356.4	282.1
Greater than 12 months, Gross Unrealized Losses	5.6	3.5
Greater than 12 months, Fair Value	105.3	103.1
Total, Gross Unrealized Losses	12.7	5.6
Total, Fair Value	461.7	385.2
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.8	0.9
12 months or less, Fair Value	139.1	99.4
Greater than 12 months, Gross Unrealized Losses	0.4	0.4
Greater than 12 months, Fair Value	22.0	22.8
Total, Gross Unrealized Losses	2.2	1.3
Total, Fair Value	161.1	122.2
Municipal [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.8	2.1
12 months or less, Fair Value	351.7	257.5
Greater than 12 months, Gross Unrealized Losses	6.2	5.0
Greater than 12 months, Fair Value	130.5	133.1
Total, Gross Unrealized Losses	12.0	7.1
Total, Fair Value	482.2	390.6
Municipal [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	1.0
Total, Fair Value	1.0
Corporate [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	40.5	5.6
12 months or less, Fair Value	2,106.7	799.6
Greater than 12 months, Gross Unrealized Losses	27.5	12.3
Greater than 12 months, Fair Value	487.8	481.3
Total, Gross Unrealized Losses	68.0	17.9
Total, Fair Value	2,594.5	1,280.9
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.7	1.7
12 months or less, Fair Value	97.7	53.2
Greater than 12 months, Gross Unrealized Losses	6.4	6.8
Greater than 12 months, Fair Value	39.0	43.5
Total, Gross Unrealized Losses	10.1	8.5
Total, Fair Value	136.7	96.7
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.6	2.0
12 months or less, Fair Value	507.4	272.9
Greater than 12 months, Gross Unrealized Losses	16.2	9.1
Greater than 12 months, Fair Value	351.5	362.4
Total, Gross Unrealized Losses	26.8	11.1
Total, Fair Value	858.9	635.3
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	6.8	0.9
12 months or less, Fair Value	405.0	139.3
Greater than 12 months, Gross Unrealized Losses	2.6	1.6
Greater than 12 months, Fair Value	61.6	63.2
Total, Gross Unrealized Losses	9.4	2.5
Total, Fair Value	466.6	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	50.3	34.5
Greater than 12 months, Fair Value	2.3	2.5
Total, Gross Unrealized Losses	1.0	0.3
Total, Fair Value	52.6	37.0
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	77.3	15.6
12 months or less, Fair Value	4,015.3	1,938.5
Greater than 12 months, Gross Unrealized Losses	64.9	38.7
Greater than 12 months, Fair Value	1,200.0	1,211.9
Total, Gross Unrealized Losses	142.2	54.3
Total, Fair Value	$ 5,215.3	$ 3,150.4