Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,459.4
Gross unrealized gains	54.8
Gross unrealized loss	(632.7)
Fixed maturities, allowance for credit losses	(9.5)	$ (8.2)	$ (8.6)	$ (9.6)
Fair Value	6,872.0
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost		7,565.8
Gross unrealized gains		54.6
Gross unrealized loss		(730.3)
Fair Value		6,881.9
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	597.1	594.1
Gross unrealized gains	3.1	1.9
Gross unrealized loss	(78.3)	(84.5)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	521.9	511.5
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,530.4	1,575.9
Gross unrealized gains	17.3	16.3
Gross unrealized loss	(173.8)	(189.8)
Fixed maturities, allowance for credit losses	(0.2)	(0.5)	(0.5)	(0.7)
Fair Value	1,373.7	1,401.9
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	5.4	4.4
Gross unrealized gains	0.1	0.0
Gross unrealized loss	(0.4)	(0.6)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	5.1	3.8
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	4,071.0	4,046.8
Gross unrealized gains	31.6	35.5
Gross unrealized loss	(341.1)	(383.8)
Fixed maturities, allowance for credit losses	(8.2)	(7.7)	$ (8.1)	$ (8.9)
Fair Value	3,753.3	3,690.8
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	11.0	9.0
Gross unrealized gains	0.1	0.1
Gross unrealized loss	(0.8)	(0.8)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	10.3	8.3
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	804.1	973.6
Gross unrealized gains	1.8	0.7
Gross unrealized loss	(8.3)	(24.5)
Fixed maturities, allowance for credit losses	(1.1)	0.0
Fair Value	796.5	949.8
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	440.4	362.0
Gross unrealized gains	0.8	0.1
Gross unrealized loss	(30.0)	(46.3)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	$ 411.2	$ 315.8