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Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule of Investments [Line Items] |  |
Amortized Cost, net | $ 5,785.4 | $ 5,652.9
Fair Value | 5,342.7 | 5,235.3
Fixed maturity securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 5,785.4 | 5,652.9
Gross Unrealized Gains | 56.7 | 62.9
Gross Unrealized Losses | 499.4 | 480.5
Fair Value | 5,342.7 | 5,235.3
Mortgage-backed securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 746.5 | 713.4
Gross Unrealized Gains | 2.9 | 4.4
Gross Unrealized Losses | 71.7 | 64.6
Fair Value | 677.7 | 653.2
Other, including U.S. Treasury securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 457.6 | 450.8
Gross Unrealized Gains | 0.4 | 0.8
Gross Unrealized Losses | 66.8 | 62.8
Fair Value | 391.2 | 388.8
Municipal bonds |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 1,282.2 | 1,333.4
Gross Unrealized Gains | 22.6 | 28.6
Gross Unrealized Losses | 97.3 | 91.9
Fair Value | 1,207.5 | 1,270.1
Foreign government bonds |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 23.1 | 23.1
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 1.2 | 1.0
Fair Value | 21.9 | 22.1
Corporate bonds |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 2,041.1 | 1,969.9
Gross Unrealized Gains | 18.5 | 23.1
Gross Unrealized Losses | 229.7 | 220.3
Fair Value | 1,829.9 | 1,772.7
Other asset-backed securities |  |
Schedule of Investments [Line Items] |  |
Amortized Cost, net | 1,234.9 | 1,162.3
Gross Unrealized Gains | 12.3 | 6.0
Gross Unrealized Losses | 32.7 | 39.9
Fair Value | $ 1,214.5 | $ 1,128.4

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