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Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Schedule of Investments [Line Items] |  |
Amortized Cost, net | $ 5,985.5 | $ 5,842.5
Fair Value | 5,666.5 | 5,387.9
Fixed maturity securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 5,985.5 | 5,842.5
Gross Unrealized Gains | 76.2 | 48.4
Gross Unrealized Losses | 395.2 | 503.0
Fair Value | 5,666.5 | 5,387.9
Mortgage-backed securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 663.9 | 827.9
Gross Unrealized Gains | 8.1 | 2.7
Gross Unrealized Losses | 39.6 | 74.8
Fair Value | 632.4 | 755.8
Other, including U.S. Treasury securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 415.5 | 426.5
Gross Unrealized Gains | 1.0 | 0.1
Gross Unrealized Losses | 56.6 | 69.0
Fair Value | 359.9 | 357.6
Municipal bonds |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 1,264.0 | 1,239.1
Gross Unrealized Gains | 22.2 | 17.5
Gross Unrealized Losses | 88.0 | 105.8
Fair Value | 1,198.2 | 1,150.8
Foreign government bonds |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 14.1 | 14.1
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.6 | 1.0
Fair Value | 13.5 | 13.1
Corporate bonds |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 1,989.9 | 1,995.2
Gross Unrealized Gains | 32.4 | 17.3
Gross Unrealized Losses | 181.6 | 230.1
Fair Value | 1,840.7 | 1,782.4
Other asset-backed securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 1,638.1 | 1,339.7
Gross Unrealized Gains | 12.5 | 10.8
Gross Unrealized Losses | 28.8 | 22.3
Fair Value | $ 1,621.8 | $ 1,328.2

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