Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 38.3	$ 8.4
12 months or less, Fair Value	1,992.4	1,086.5
Greater than 12 months, Gross Unrealized Losses	77.8	22.1
Greater than 12 months, Fair Value	1,758.5	783.3
Total, Gross Unrealized Losses	116.1	30.5
Total, Fair Value	3,750.9	1,869.8
Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	8.1
12 months or less, Fair Value	185.6
Greater than 12 months, Gross Unrealized Losses	10.0
Greater than 12 months, Fair Value	54.9
Total, Gross Unrealized Losses	18.1
Total, Fair Value	240.5
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	46.4	9.9
12 months or less, Fair Value	2,178.0	1,135.2
Greater than 12 months, Gross Unrealized Losses	87.8	28.5
Greater than 12 months, Fair Value	1,813.4	822.3
Total, Gross Unrealized Losses	134.2	38.4
Total, Fair Value	3,991.4	1,957.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	46.5	15.6
12 months or less, Fair Value	2,182.0	1,938.5
Greater than 12 months, Gross Unrealized Losses	88.1	38.7
Greater than 12 months, Fair Value	1,831.4	1,211.9
Total, Gross Unrealized Losses	134.6	54.3
Total, Fair Value	4,013.4	3,150.4
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	5.7
12 months or less, Fair Value	4.0	803.3
Greater than 12 months, Gross Unrealized Losses	0.3	10.2
Greater than 12 months, Fair Value	18.0	389.6
Total, Gross Unrealized Losses	0.4	15.9
Total, Fair Value	22.0	1,192.9
US Treasury and Government Agencies [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.1	1.1
12 months or less, Fair Value	66.0	146.3
Greater than 12 months, Gross Unrealized Losses	6.1	2.1
Greater than 12 months, Fair Value	210.9	62.3
Total, Gross Unrealized Losses	7.2	3.2
Total, Fair Value	276.9	208.6
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	2.0	0.1
Greater than 12 months, Fair Value	0.8	0.6
Total, Fair Value	2.8	0.7
Municipal [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.8	1.7
12 months or less, Fair Value	110.0	183.8
Greater than 12 months, Gross Unrealized Losses	9.0	4.1
Greater than 12 months, Fair Value	248.0	101.2
Total, Gross Unrealized Losses	9.8	5.8
Total, Fair Value	358.0	285.0
Municipal [Member] | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, 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	30.0	3.0
12 months or less, Fair Value	1,277.9	386.2
Greater than 12 months, Gross Unrealized Losses	43.9	10.8
Greater than 12 months, Fair Value	781.6	395.9
Total, Gross Unrealized Losses	73.9	13.8
Total, Fair Value	2,059.5	782.1
Corporate [Member] | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	8.1	1.5
12 months or less, Fair Value	185.6	48.7
Greater than 12 months, Gross Unrealized Losses	10.0	6.4
Greater than 12 months, Fair Value	54.0	39.0
Total, Gross Unrealized Losses	18.1	7.9
Total, Fair Value	239.6	87.7
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.6	1.4
12 months or less, Fair Value	201.2	196.3
Greater than 12 months, Gross Unrealized Losses	12.1	3.5
Greater than 12 months, Fair Value	323.7	159.0
Total, Gross Unrealized Losses	14.7	4.9
Total, Fair Value	524.9	355.3
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.4	0.9
12 months or less, Fair Value	293.0	139.2
Greater than 12 months, Gross Unrealized Losses	6.4	1.6
Greater than 12 months, Fair Value	175.5	61.9
Total, Gross Unrealized Losses	9.8	2.5
Total, Fair Value	468.5	201.1
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	0.3
12 months or less, Fair Value	42.3	34.6
Greater than 12 months, Gross Unrealized Losses	0.3
Greater than 12 months, Fair Value	18.0	2.4
Total, Gross Unrealized Losses	0.7	0.3
Total, Fair Value	$ 60.3	$ 37.0