Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 25.6	$ 62.1
12 months or less, Fair Value	1,941.1	2,419.2
Greater than 12 months, Gross Unrealized Losses	26.3	20.9
Greater than 12 months, Fair Value	533.5	215.9
Total, Gross Unrealized Losses	51.9	83.0
Total, Fair Value	2,474.6	2,635.1
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	24.0	60.2
12 months or less, Fair Value	1,896.4	2,356.9
Greater than 12 months, Gross Unrealized Losses	18.7	8.7
Greater than 12 months, Fair Value	481.1	134.1
Total, Gross Unrealized Losses	42.7	68.9
Total, Fair Value	2,377.5	2,491.0
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		1.2
12 months or less, Fair Value		46.0
Greater than 12 months, Gross Unrealized Losses		12.2
Greater than 12 months, Fair Value		81.8
Total, Gross Unrealized Losses		13.4
Total, Fair Value		127.8
US Treasury and Government Agencies [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.1	5.1
12 months or less, Fair Value	213.1	165.9
Greater than 12 months, Gross Unrealized Losses	1.2
Greater than 12 months, Fair Value	40.8
Total, Gross Unrealized Losses	4.3	5.1
Total, Fair Value	253.9	165.9
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.1	0.5
12 months or less, Fair Value	112.0	55.0
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	7.3	1.8
Total, Gross Unrealized Losses	1.2	0.5
Total, Fair Value	119.3	56.8
Municipal [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.3	7.2
12 months or less, Fair Value	187.3	268.4
Greater than 12 months, Gross Unrealized Losses	2.5	2.0
Greater than 12 months, Fair Value	57.9	29.3
Total, Gross Unrealized Losses	4.8	9.2
Total, Fair Value	245.2	297.7
Corporate [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.1	30.6
12 months or less, Fair Value	792.5	1,081.0
Greater than 12 months, Gross Unrealized Losses	10.2	5.0
Greater than 12 months, Fair Value	229.9	64.2
Total, Gross Unrealized Losses	20.3	35.6
Total, Fair Value	1,022.4	1,145.2
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.3	1.2
12 months or less, Fair Value	33.7	45.9
Greater than 12 months, Gross Unrealized Losses	7.6	12.2
Greater than 12 months, Fair Value	52.4	81.8
Total, Gross Unrealized Losses	8.9	13.4
Total, Fair Value	86.1	127.7
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	6.0	12.1
12 months or less, Fair Value	440.5	570.0
Greater than 12 months, Gross Unrealized Losses	3.4	1.5
Greater than 12 months, Fair Value	95.1	29.0
Total, Gross Unrealized Losses	9.4	13.6
Total, Fair Value	535.6	599.0
Residential Mortgage Backed [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value		0.1
Total, Fair Value		0.1
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.3	4.1
12 months or less, Fair Value	118.5	187.5
Greater than 12 months, Gross Unrealized Losses	1.3
Greater than 12 months, Fair Value	50.1	6.3
Total, Gross Unrealized Losses	2.6	4.1
Total, Fair Value	168.6	193.8
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	0.6
12 months or less, Fair Value	32.5	29.1
Greater than 12 months, Gross Unrealized Losses		0.2
Greater than 12 months, Fair Value		3.5
Total, Gross Unrealized Losses	0.1	0.8
Total, Fair Value	32.5	32.6
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	25.3	61.4
12 months or less, Fair Value	1,930.1	2,402.9
Greater than 12 months, Gross Unrealized Losses	26.3	20.9
Greater than 12 months, Fair Value	533.5	215.9
Total, Gross Unrealized Losses	51.6	82.3
Total, Fair Value	2,463.6	2,618.8
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3	0.7
12 months or less, Fair Value	11.0	16.3
Total, Gross Unrealized Losses	0.3	0.7
Total, Fair Value	$ 11.0	$ 16.3