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Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023
Common equity securities |  |
Cost or Amortized Cost | $ 483.5 | $ 483.5
Gross Unrealized Gains | 86.0 | 62.3
Gross Unrealized Losses | (1.2) | (1.2)
Net Foreign Currency Gains (Losses) | (6.8) | (6.2)
Carrying Value | 561.5 | 538.4
Other long-term investments |  |
Cost or Amortized Cost | 1,642.2 | 1,655.7
Gross Unrealized Gains | 466.7 | 446.3
Other Long-Term Investments, Unrealized Loss | (101.1) | (94.5)
Net Foreign Currency Gains (Losses) | (15.9) | (9.3)
Carrying Value | 1,991.9 | 1,998.2
MediaAlpha |  |
Investment in MediaAlpha |  |
Cost or Amortized Cost | 59.2 | 59.2
Gross Unrealized Gains | 406.4 | 195.7
Gross Unrealized Losses | 0.0 | 0.0
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | $ 465.6 | $ 254.9

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