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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | $ 1,497.7 | $ 2,025.6
Gross Unrealized Gains | 7.7 | 6.1
Gross Unrealized Losses | (16.7) | (67.3)
Fair Market Value | 1,488.7 | 1,964.4
Fixed income investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1,207.7 | 1,527.0
Gross Unrealized Gains | 6.2 | 4.7
Gross Unrealized Losses | (14.0) | (46.0)
Fair Market Value | 1,199.9 | 1,485.7
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 262.9 | 248.7
Gross Unrealized Gains | 0.6 | 0.5
Gross Unrealized Losses | (2.2) | (3.7)
Fair Market Value | 261.3 | 245.5
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1.6 | 3.3
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | (0.2)
Fair Market Value | 1.6 | 3.1
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 155.5 | 178.8
Gross Unrealized Gains | 0.5 | 0.7
Gross Unrealized Losses | (4.9) | (8.0)
Fair Market Value | 151.1 | 171.5
High yield loans |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 101.7 | 90.8
Gross Unrealized Gains | 0.9 | 1.3
Gross Unrealized Losses | (0.2) | 0.0
Fair Market Value | 102.4 | 92.1
Non-U.S. government-backed corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 2.8 | 8.6
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | (0.3)
Fair Market Value | 2.8 | 8.3
Non-U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 24.4 | 35.8
Gross Unrealized Gains | 0.1 | 0.1
Gross Unrealized Losses | (0.1) | (1.1)
Fair Market Value | 24.4 | 34.8
Asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 624.6 | 936.0
Gross Unrealized Gains | 4.1 | 2.1
Gross Unrealized Losses | (3.5) | (29.9)
Fair Market Value | 625.2 | 908.2
Agency mortgage-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 34.2 | 25.0
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (3.1) | (2.8)
Fair Market Value | 31.1 | 22.2
Short-term investments — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1.0 | 2.1
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 1.0 | 2.1
Catastrophe bonds, trading |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 1.0 | 1.6
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Market Value | 1.0 | 1.6
Privately-held investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 288.0 | 494.9
Gross Unrealized Gains | 1.5 | 1.4
Gross Unrealized Losses | (2.7) | (21.3)
Fair Market Value | 286.8 | 475.0
Commercial mortgage loans |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 80.9 | 293.2
Gross Unrealized Gains | 0.5 | 1.0
Gross Unrealized Losses | (1.7) | (19.3)
Fair Market Value | 79.7 | 274.9
Middle market loans and other private debt |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 61.6 | 85.9
Gross Unrealized Gains | 0.1 | 0.0
Gross Unrealized Losses | (0.7) | (1.1)
Fair Market Value | 61.0 | 84.8
Asset-backed securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 126.7 | 83.1
Gross Unrealized Gains | 0.9 | 0.4
Gross Unrealized Losses | 0.0 | (0.6)
Fair Market Value | 127.6 | 82.9
Global corporate securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost | 18.8 | 14.7
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.3) | (0.3)
Fair Market Value | $ 18.5 | 14.4
Short-term investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Cost or Amortized Cost |  | 18.0
Gross Unrealized Gains |  | 0.0
Gross Unrealized Losses |  | 0.0
Fair Market Value |  | $ 18.0

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