Investments - Schedule of Estimated Fair Values and Gross Unrealized Losses (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Less than 12 months
Fair Value	$ 1,363.5	$ 358.8
Unrealized Losses	(306.5)	(5.8)
12 months or greater
Fair Value	27.3	441.6
Unrealized Losses	(5.7)	(27.6)
Fair Value	1,390.8	800.4
Unrealized Losses	(312.2)	(33.4)
U.S. Government and government agencies
Less than 12 months
Fair Value	0.2	0.3
Unrealized Losses	0.0	0.0
12 months or greater
Fair Value	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	0.2	0.3
Unrealized Losses	0.0	0.0
States, municipalities and political subdivisions
Less than 12 months
Fair Value	28.9	2.0
Unrealized Losses	(0.4)	0.0
12 months or greater
Fair Value	0.0	0.0
Unrealized Losses	0.0	0.0
Fair Value	28.9	2.0
Unrealized Losses	(0.4)	0.0
Residential mortgage-backed securities
Less than 12 months
Fair Value	14.6	2.3
Unrealized Losses	(1.6)	0.0
12 months or greater
Fair Value	4.0	8.2
Unrealized Losses	(0.6)	(0.6)
Fair Value	18.6	10.5
Unrealized Losses	(2.2)	(0.6)
Commercial mortgage-backed securities
Less than 12 months
Fair Value	71.9	58.1
Unrealized Losses	(13.5)	(0.6)
12 months or greater
Fair Value	0.2	0.2
Unrealized Losses	0.0	0.0
Fair Value	72.1	58.3
Unrealized Losses	(13.5)	(0.6)
Asset-backed securities
Less than 12 months
Fair Value	482.5	126.5
Unrealized Losses	(109.1)	(1.5)
12 months or greater
Fair Value	11.0	255.8
Unrealized Losses	(2.2)	(15.5)
Fair Value	493.5	382.3
Unrealized Losses	(111.3)	(17.0)
Corporate and other
Less than 12 months
Fair Value	765.4	169.6
Unrealized Losses	(181.9)	(3.7)
12 months or greater
Fair Value	12.1	177.4
Unrealized Losses	(2.9)	(11.5)
Fair Value	777.5	347.0
Unrealized Losses	$ (184.8)	$ (15.2)