Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,484.3	$ 7,295.0
Gross unrealized gains	26.1	14.8
Gross unrealized loss	(828.7)	(889.5)
Fixed maturities, allowance for credit losses	(12.6)	(10.7)
Fair Value	6,669.1	6,409.6
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	682.3	588.6
Gross unrealized gains	2.6	0.6
Gross unrealized loss	(91.9)	(102.4)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	593.0	486.8
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,441.9	1,457.3
Gross unrealized gains	1.5	1.6
Gross unrealized loss	(223.3)	(225.4)
Fixed maturities, allowance for credit losses	(0.2)	(0.3)
Fair Value	1,219.9	1,233.2
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	6.5	6.5
Gross unrealized gains	0.4	0.3
Gross unrealized loss	(0.2)	(0.2)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	6.7	6.6
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	4,069.6	4,038.3
Gross unrealized gains	18.2	8.9
Gross unrealized loss	(478.3)	(518.8)
Fixed maturities, allowance for credit losses	(9.9)	(8.8)
Fair Value	3,599.6	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	0.4	0.1
Gross unrealized loss	(0.1)	(1.0)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	10.1	8.9
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	876.1	747.8
Gross unrealized gains	2.3	2.5
Gross unrealized loss	(7.6)	(7.2)
Fixed maturities, allowance for credit losses	(2.5)	(1.6)
Fair Value	868.3	741.5
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	398.1	446.7
Gross unrealized gains	0.7	0.8
Gross unrealized loss	(27.3)	(34.5)
Fixed maturities, allowance for credit losses	0.0	0.0
Fair Value	$ 371.5	$ 413.0