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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

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