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Investments - Debt Securities Unrealized Loss Positions (Details) - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
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Fair value |  |
Debt securities, Fair Value, 12 Months or Less | $ 911.8 | $ 1,670.9
Debt securities, Fair Value, Greater than 12 Months | 335.4 | 364.7
Debt securities, Fair Value | 1,247.2 | 2,035.6
Gross unrealized losses |  |
Debt securities, Gross Unrealized Losses, 12 Months or Less | (9.3) | (23.4)
Debt securities, Gross Unrealized Losses, Greater than 12 Months | (12.2) | (16.8)
Debt securities, Gross Unrealized Losses | (21.5) | (40.2)
Asset-backed securities |  |
Fair value |  |
Debt securities, Fair Value, 12 Months or Less | 246.2 | 180.4
Debt securities, Fair Value, Greater than 12 Months | 37.6 | 23.8
Debt securities, Fair Value | 283.8 | 204.2
Gross unrealized losses |  |
Debt securities, Gross Unrealized Losses, 12 Months or Less | (1.1) | (1.8)
Debt securities, Gross Unrealized Losses, Greater than 12 Months | (2.2) | (2.4)
Debt securities, Gross Unrealized Losses | (3.3) | (4.2)
Residential mortgage-backed securities |  |
Fair value |  |
Debt securities, Fair Value, 12 Months or Less | 195.7 | 437.4
Debt securities, Fair Value, Greater than 12 Months | 223.2 | 223.9
Debt securities, Fair Value | 418.9 | 661.3
Gross unrealized losses |  |
Debt securities, Gross Unrealized Losses, 12 Months or Less | (2.1) | (7.9)
Debt securities, Gross Unrealized Losses, Greater than 12 Months | (8.8) | (12.4)
Debt securities, Gross Unrealized Losses | (10.9) | (20.3)
Commercial mortgage-backed securities |  |
Fair value |  |
Debt securities, Fair Value, 12 Months or Less | 45.1 | 67.4
Debt securities, Fair Value, Greater than 12 Months | 6.0 | 7.5
Debt securities, Fair Value | 51.1 | 74.9
Gross unrealized losses |  |
Debt securities, Gross Unrealized Losses, 12 Months or Less | (0.2) | (0.7)
Debt securities, Gross Unrealized Losses, Greater than 12 Months | (0.4) | (0.4)
Debt securities, Gross Unrealized Losses | (0.6) | (1.1)
Corporate debt securities |  |
Fair value |  |
Debt securities, Fair Value, 12 Months or Less | 312.4 | 738.5
Debt securities, Fair Value, Greater than 12 Months | 25.1 | 46.8
Debt securities, Fair Value | 337.5 | 785.3
Gross unrealized losses |  |
Debt securities, Gross Unrealized Losses, 12 Months or Less | (5.2) | (10.0)
Debt securities, Gross Unrealized Losses, Greater than 12 Months | (0.2) | (0.4)
Debt securities, Gross Unrealized Losses | (5.4) | (10.4)
U.S. government and government agency |  |
Fair value |  |
Debt securities, Fair Value, 12 Months or Less | 112.4 | 247.2
Debt securities, Fair Value, Greater than 12 Months | 43.5 | 62.7
Debt securities, Fair Value | 155.9 | 309.9
Gross unrealized losses |  |
Debt securities, Gross Unrealized Losses, 12 Months or Less | (0.7) | (3.0)
Debt securities, Gross Unrealized Losses, Greater than 12 Months | (0.6) | (1.2)
Debt securities, Gross Unrealized Losses | $ (1.3) | $ (4.2)

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