Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 1.2
12 months or less, Fair Value	44.5
Greater than 12 months, Gross Unrealized Losses	40.8
Greater than 12 months, Fair Value	2,031.6
Total, Gross Unrealized Losses	42.0
Total, Fair Value	2,076.1
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.9	$ 38.3
12 months or less, Fair Value	27.4	1,992.4
Greater than 12 months, Gross Unrealized Losses	34.3	77.8
Greater than 12 months, Fair Value	1,955.9	1,758.5
Total, Gross Unrealized Losses	35.2	116.1
Total, Fair Value	1,983.3	3,750.9
Below Investment Grade [Member]
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
12 months or less, Fair Value		2,178.0
Greater than 12 months, Gross Unrealized Losses		87.8
Greater than 12 months, Fair Value		1,813.4
Total, Gross Unrealized Losses		134.2
Total, Fair Value		3,991.4
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.1
12 months or less, Fair Value		4.0
Greater than 12 months, Gross Unrealized Losses		0.3
Greater than 12 months, Fair Value		18.0
Total, Gross Unrealized Losses		0.4
Total, Fair Value		22.0
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		46.5
12 months or less, Fair Value		2,182.0
Greater than 12 months, Gross Unrealized Losses		88.1
Greater than 12 months, Fair Value		1,831.4
Total, Gross Unrealized Losses		134.6
Total, Fair Value		4,013.4
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
12 months or less, Fair Value		66.0
Greater than 12 months, Gross Unrealized Losses	2.5	6.1
Greater than 12 months, Fair Value	158.4	210.9
Total, Gross Unrealized Losses	2.5	7.2
Total, Fair Value	158.4	276.9
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value		2.0
Greater than 12 months, Fair Value	0.8	0.8
Total, Fair Value	0.8	2.8
Municipal [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	9.7	110.0
Greater than 12 months, Gross Unrealized Losses	3.4	9.0
Greater than 12 months, Fair Value	192.3	248.0
Total, Gross Unrealized Losses	3.4	9.8
Total, Fair Value	202.0	358.0
Municipal [Member] | Below Investment Grade [Member]
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	0.9	30.0
12 months or less, Fair Value	15.3	1,277.9
Greater than 12 months, Gross Unrealized Losses	19.7	43.9
Greater than 12 months, Fair Value	993.2	781.6
Total, Gross Unrealized Losses	20.6	73.9
Total, Fair Value	1,008.5	2,059.5
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3	8.1
12 months or less, Fair Value	17.1	185.6
Greater than 12 months, Gross Unrealized Losses	6.5	10.0
Greater than 12 months, Fair Value	75.7	54.0
Total, Gross Unrealized Losses	6.8	18.1
Total, Fair Value	92.8	239.6
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		2.6
12 months or less, Fair Value	2.1	201.2
Greater than 12 months, Gross Unrealized Losses	6.1	12.1
Greater than 12 months, Fair Value	352.6	323.7
Total, Gross Unrealized Losses	6.1	14.7
Total, Fair Value	354.7	524.9
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		3.4
12 months or less, Fair Value	0.3	293.0
Greater than 12 months, Gross Unrealized Losses	2.4	6.4
Greater than 12 months, Fair Value	226.7	175.5
Total, Gross Unrealized Losses	2.4	9.8
Total, Fair Value	227.0	468.5
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.4
12 months or less, Fair Value		42.3
Greater than 12 months, Gross Unrealized Losses	0.2	0.3
Greater than 12 months, Fair Value	31.9	18.0
Total, Gross Unrealized Losses	0.2	0.7
Total, Fair Value	$ 31.9	$ 60.3