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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | $ 4,330.0 | $ 4,131.3
Gross unrealized gains | 17.3 | 3.6
Gross unrealized losses | (221.8) | (338.6)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (2.9) | (7.7)
Fair market value | 4,122.6 | 3,788.6
Total short-term investments — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | 0.0 | (0.4)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0
Fixed maturities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 4,438.3 | 4,183.7
Gross unrealized gains | 17.5 | 3.6
Gross unrealized losses | (221.8) | (339.0)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (2.9) | (7.7)
Fair market value | 4,231.1 | 3,840.6
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 1,224.9 | 1,003.7
Gross unrealized gains | 4.4 | 0.0
Gross unrealized losses | (26.7) | (50.7)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0
Fair market value | 1,202.6 | 953.0
U.S. agency |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 7.5 | 9.2
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | (0.3) | (0.4)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0
Fair market value | 7.2 | 8.8
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 133.6 | 159.9
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | (5.0) | (9.4)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (0.5) | (1.0)
Fair market value | 128.1 | 149.5
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 2,051.1 | 2,016.9
Gross unrealized gains | 12.1 | 3.5
Gross unrealized losses | (101.5) | (169.1)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | (2.4) | (6.3)
Fair market value | 1,959.3 | 1,845.0
Non-U.S. government-backed corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 106.5 | 119.4
Gross unrealized gains | 0.1 | 0.0
Gross unrealized losses | (5.9) | (8.8)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | (0.2)
Fair market value | 100.7 | 110.4
Non-U.S government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 279.9 | 225.2
Gross unrealized gains | 0.6 | 0.1
Gross unrealized losses | (6.7) | (11.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | (0.2)
Fair market value | 273.8 | 213.6
Agency commercial mortgage-backed securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 6.6 | 6.6
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | (0.8) | (1.0)
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 | 0.0
Fair market value | 5.8 | 5.6
Collateralized Mortgage-Backed Securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 519.9 | 590.4
Gross unrealized gains |  | 0.0
Gross unrealized losses |  | (87.7)
Debt Securities, Available-for-Sale, Allowance for Credit Loss |  | 0.0
Fair market value | 445.1 | 502.7
Agency residential mortgage-backed securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 519.9 |
Gross unrealized gains | 0.1 |
Gross unrealized losses | (74.9) |
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 |
Fair market value | 445.1 |
Total short-term investments — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 93.6 | 52.4
Fair market value | 93.6 | $ 52.0
Privately-held investments, Available for Sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fixed income securities, available for sale amortized cost | 14.7 |
Gross unrealized gains | 0.2 |
Gross unrealized losses | 0.0 |
Debt Securities, Available-for-Sale, Allowance for Credit Loss | 0.0 |
Fair market value | $ 14.9 |

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