Investments - Schedule of Fixed Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,824.0
Unrealized Losses	(312.2)	$ (33.4)
Fair Value	3,753.0	4,028.9
Total fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,824.0	3,745.3
Unrealized Gains	241.2	317.0
Unrealized Losses	(312.2)	(33.4)
Fair Value	3,753.0	4,028.9
U.S. Government and government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7.5	7.0
Unrealized Gains	1.3	0.7
Unrealized Losses	0.0	0.0
Fair Value	8.8	7.7
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	391.8	405.4
Unrealized Gains	36.0	34.7
Unrealized Losses	(0.4)	0.0
Fair Value	427.4	440.1
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	60.0	63.0
Unrealized Gains	3.2	4.5
Unrealized Losses	(2.2)	(0.6)
Fair Value	61.0	66.9
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	112.5	108.2
Unrealized Gains	0.7	1.8
Unrealized Losses	(13.5)	(0.6)
Fair Value	99.7	109.4
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	651.8	592.6
Unrealized Gains	0.7	2.2
Unrealized Losses	(111.3)	(17.0)
Fair Value	541.2	577.8
Corporate and other
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,600.4	2,569.1
Unrealized Gains	199.3	273.1
Unrealized Losses	(184.8)	(15.2)
Fair Value	$ 2,614.9	$ 2,827.0