Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,372.2	$ 7,295.0
Gross unrealized gains	17.4	14.8
Gross unrealized loss	(821.7)	(889.5)
Fixed maturities, allowance for credit losses	(9.3)	(10.7)
Fair Value	6,558.6	6,409.6
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	601.0	588.6
Gross unrealized gains	1.7	0.6
Gross unrealized loss	(90.9)	(102.4)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	511.8	486.8
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,447.4	1,457.3
Gross unrealized gains	2.1	1.6
Gross unrealized loss	(217.1)	(225.4)
Fixed maturities, allowance for credit losses	(0.2)	(0.3)
Fair Value	1,232.2	1,233.2
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	6.6	6.5
Gross unrealized gains	0.2	0.3
Gross unrealized loss	(0.2)	(0.2)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	6.6	6.6
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	4,087.4	4,038.3
Gross unrealized gains	11.0	8.9
Gross unrealized loss	(476.0)	(518.8)
Fixed maturities, allowance for credit losses	(7.1)	(8.8)
Fair Value	3,615.3	3,519.6
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	9.8	9.8
Gross unrealized gains	0.1	0.1
Gross unrealized loss	(0.9)	(1.0)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	9.0	8.9
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	796.3	747.8
Gross unrealized gains	1.8	2.5
Gross unrealized loss	(6.8)	(7.2)
Fixed maturities, allowance for credit losses	(2.0)	(1.6)
Fair Value	789.3	741.5
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	423.7	446.7
Gross unrealized gains	0.5	0.8
Gross unrealized loss	(29.8)	(34.5)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	$ 394.4	$ 413.0