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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Fixed maturities
Amortized Cost	$ 2,283.6		$ 2,521.5
Gross Unrealized Gains	86.9		65.5
Gross Unrealized Losses	1.3		1.4
Allowance for Credit Losses	1.0	$ 0.5	0.2	$ 4.4	$ 2.5	$ 2.8
Fair Value	2,368.2		2,585.4
U.S. Governments
Fixed maturities
Amortized Cost	259.8		357.7
Gross Unrealized Gains	5.2		4.5
Gross Unrealized Losses	0.1		0.0
Allowance for Credit Losses	0.0		0.0
Fair Value	264.9		362.2
Foreign Governments
Fixed maturities
Amortized Cost	7.1		27.9
Gross Unrealized Gains	0.3		2.6
Gross Unrealized Losses	0.0		0.0
Allowance for Credit Losses	0.0		0.2
Fair Value	7.4		30.3
Obligations of states and political subdivisions
Fixed maturities
Amortized Cost	82.2		92.4
Gross Unrealized Gains	3.0		2.0
Gross Unrealized Losses	0.0		0.0
Allowance for Credit Losses	0.0		0.0
Fair Value	85.2		94.4
Corporate bonds
Fixed maturities
Amortized Cost	1,206.7		1,185.0
Gross Unrealized Gains	39.9		28.9
Gross Unrealized Losses	0.9		0.8
Allowance for Credit Losses	1.0		0.0
Fair Value	1,244.7		1,213.1
Commercial mortgage-backed securities
Fixed maturities
Amortized Cost	275.1		270.9
Gross Unrealized Gains	16.9		10.1
Gross Unrealized Losses	0.3		0.5
Allowance for Credit Losses	0.0		0.0
Fair Value	291.7		280.5
Residential mortgage-backed securities
Fixed maturities
Amortized Cost	204.9		235.2
Gross Unrealized Gains	13.7		13.6
Gross Unrealized Losses	0.0		0.0
Allowance for Credit Losses	0.0		0.0
Fair Value	218.6		248.8
Asset-backed securities
Fixed maturities
Amortized Cost	104.9		140.4
Gross Unrealized Gains	3.5		1.7
Gross Unrealized Losses	0.0		0.1
Allowance for Credit Losses	0.0		0.0
Fair Value	108.4		142.0
Collateralized loan obligations
Fixed maturities
Amortized Cost	142.9		212.0
Gross Unrealized Gains	4.4		2.1
Gross Unrealized Losses	0.0		0.0
Allowance for Credit Losses	0.0		0.0
Fair Value	$ 147.3		$ 214.1