Investments - Schedule of Fixed Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,745.3
Unrealized Losses	(33.4)	$ (117.4)
Fair Value	4,028.9	3,391.6
Total fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,745.3	3,477.5
Unrealized Gains	317.0	31.5
Unrealized Losses	(33.4)	(117.4)
Fair Value	4,028.9	3,391.6
U.S. Government and government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7.0	24.7
Unrealized Gains	0.7	0.7
Unrealized Losses	0.0	0.0
Fair Value	7.7	25.4
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	405.4	413.7
Unrealized Gains	34.7	9.6
Unrealized Losses	0.0	(1.4)
Fair Value	440.1	421.9
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	63.0	92.6
Unrealized Gains	4.5	3.1
Unrealized Losses	(0.6)	(1.3)
Fair Value	66.9	94.4
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	108.2	94.7
Unrealized Gains	1.8	0.3
Unrealized Losses	(0.6)	(1.1)
Fair Value	109.4	93.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	592.6	540.8
Unrealized Gains	2.2	0.8
Unrealized Losses	(17.0)	(30.1)
Fair Value	577.8	511.5
Corporate and other
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,569.1	2,311.0
Unrealized Gains	273.1	17.0
Unrealized Losses	(15.2)	(83.5)
Fair Value	$ 2,827.0	$ 2,244.5

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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