Investments - Schedule of Estimated Fair Values and Gross Unrealized Losses (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fixed maturity securities
Less than 12 months
Fair Value	$ 656.3	$ 2,086.6
Unrealized Losses	(33.9)	(105.1)
12 months or greater
Fair Value	103.2	271.4
Unrealized Losses	(8.9)	(12.3)
Fair Value	759.5	2,358.0
Unrealized Losses	(42.8)	(117.4)
U.S. Government and government agencies
Less than 12 months
Fair Value	0.1	5.0
Unrealized Losses	0.0	0.0
12 months or greater
Fair Value	0.0	3.3
Unrealized Losses	0.0	0.0
Fair Value	0.1	8.3
Unrealized Losses	0.0	0.0
States, municipalities and political subdivisions
Less than 12 months
Fair Value	5.7	117.2
Unrealized Losses	0.0	(1.3)
12 months or greater
Fair Value	0.0	1.9
Unrealized Losses	0.0	(0.1)
Fair Value	5.7	119.1
Unrealized Losses	0.0	(1.4)
Residential mortgage-backed securities
Less than 12 months
Fair Value	11.6	22.4
Unrealized Losses	(0.5)	(1.2)
12 months or greater
Fair Value	2.6	5.7
Unrealized Losses	(0.1)	(0.1)
Fair Value	14.2	28.1
Unrealized Losses	(0.6)	(1.3)
Commercial mortgage-backed securities
Less than 12 months
Fair Value	0.9	57.8
Unrealized Losses	0.0	(1.1)
12 months or greater
Fair Value	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	0.9	57.8
Unrealized Losses	0.0	(1.1)
Asset-backed securities
Less than 12 months
Fair Value	362.3	466.0
Unrealized Losses	(7.6)	(29.6)
12 months or greater
Fair Value	61.0	5.9
Unrealized Losses	(5.0)	(0.5)
Fair Value	423.3	471.9
Unrealized Losses	(12.6)	(30.1)
Corporate and other
Less than 12 months
Fair Value	275.7	1,418.2
Unrealized Losses	(25.8)	(71.9)
12 months or greater
Fair Value	39.6	254.6
Unrealized Losses	(3.8)	(11.6)
Fair Value	315.3	1,672.8
Unrealized Losses	$ (29.6)	$ (83.5)

Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months [Abstract]

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Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, More than 12 Months [Abstract]

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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