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Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, at amortized cost | $ 7,331.7 | $ 7,295.0
Gross unrealized gains | 32.3 | 14.8
Gross unrealized loss | (728.0) | (889.5)
Fixed maturities, allowance for credit losses | (15.7) | (10.7)
Fair Value | 6,620.3 | 6,409.6
U.S. Government and Government Agencies and Authorities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, at amortized cost | 673.4 | 588.6
Gross unrealized gains | 3.2 | 0.6
Gross unrealized loss | (84.7) | (102.4)
Fixed maturities, allowance for credit losses | 0.0 | 0.0
Fair Value | 591.9 | 486.8
States and Political Subdivisions |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, at amortized cost | 1,435.6 | 1,457.3
Gross unrealized gains | 2.2 | 1.6
Gross unrealized loss | (194.5) | (225.4)
Fixed maturities, allowance for credit losses | (0.1) | (0.3)
Fair Value | 1,243.2 | 1,233.2
Foreign Governments |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, at amortized cost | 12.1 | 6.5
Gross unrealized gains | 0.2 | 0.3
Gross unrealized loss | (0.3) | (0.2)
Fixed maturities, allowance for credit losses | 0.0 | 0.0
Fair Value | 12.0 | 6.6
Corporate Bonds and Notes |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, at amortized cost | 3,982.7 | 4,038.3
Gross unrealized gains | 22.1 | 8.9
Gross unrealized loss | (417.2) | (518.8)
Fixed maturities, allowance for credit losses | (13.4) | (8.8)
Fair Value | 3,574.2 | 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 | 0.5 | 0.1
Gross unrealized loss | 0.0 | (1.0)
Fixed maturities, allowance for credit losses | 0.0 | 0.0
Fair Value | 10.3 | 8.9
Collateralized Loan Obligations |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, at amortized cost | 823.8 | 747.8
Gross unrealized gains | 2.8 | 2.5
Gross unrealized loss | (6.5) | (7.2)
Fixed maturities, allowance for credit losses | (2.2) | (1.6)
Fair Value | 817.9 | 741.5
Other Mortgage- and Asset-backed |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, at amortized cost | 394.3 | 446.7
Gross unrealized gains | 1.3 | 0.8
Gross unrealized loss | (24.8) | (34.5)
Fixed maturities, allowance for credit losses | 0.0 | 0.0
Fair Value | $ 370.8 | $ 413.0

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