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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, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | $ 5,147.0 | $ 4,438.3
Gross Unrealized Gains | 14.2 | 17.5
Gross Unrealized Losses | (181.9) | (221.8)
Allowance for Credit Losses | (1.0) | (2.9)
Fair Market Value | 4,978.3 | 4,231.1
Fixed income investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 4,861.1 | 4,330.0
Gross Unrealized Gains | 14.0 | 17.3
Gross Unrealized Losses | (181.9) | (221.8)
Allowance for Credit Losses | (1.0) | (2.9)
Fair Market Value | 4,692.2 | 4,122.6
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1,504.8 | 1,224.9
Gross Unrealized Gains | 1.5 | 4.4
Gross Unrealized Losses | (25.7) | (26.7)
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | 1,480.6 | 1,202.6
U.S. agency |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 7.5 | 7.5
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.3) | (0.3)
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | 7.2 | 7.2
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 84.2 | 133.6
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (1.9) | (5.0)
Allowance for Credit Losses | 0.0 | (0.5)
Fair Market Value | 82.3 | 128.1
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 2,045.3 | 2,051.1
Gross Unrealized Gains | 8.5 | 12.1
Gross Unrealized Losses | (66.4) | (101.5)
Allowance for Credit Losses | (1.0) | (2.4)
Fair Market Value | 1,986.4 | 1,959.3
Non-U.S. government-backed corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 132.9 | 106.5
Gross Unrealized Gains | 0.8 | 0.1
Gross Unrealized Losses | (2.4) | (5.9)
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | 131.3 | 100.7
Non-U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 248.2 | 279.9
Gross Unrealized Gains | 1.0 | 0.6
Gross Unrealized Losses | (2.4) | (6.7)
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | 246.8 | 273.8
Asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 232.4 |
Gross Unrealized Gains | 2.2 |
Gross Unrealized Losses | (0.1) |
Allowance for Credit Losses | 0.0 |
Fair Market Value | 234.5 |
Agency commercial mortgage-backed securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 5.0 | 6.6
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.6) | (0.8)
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | 4.4 | 5.8
Agency residential mortgage-backed securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 600.8 | 519.9
Gross Unrealized Gains | 0.0 | 0.1
Gross Unrealized Losses | (82.1) | (74.9)
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | 518.7 | 445.1
Short-term investments — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 261.9 | 93.6
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | 261.9 | 93.6
Privately-held investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 24.0 | 14.7
Gross Unrealized Gains | 0.2 | 0.2
Gross Unrealized Losses | 0.0 | 0.0
Allowance for Credit Losses | 0.0 | 0.0
Fair Market Value | $ 24.2 | $ 14.9

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