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Investment Securities - Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021
Common equity securities |  |
Investment securities |  |
Cost or Amortized Cost | $ 660.6 | $ 236.3
Gross Unrealized Gains | 26.7 | 16.1
Gross Unrealized Losses | (8.4) | 0.0
Net Foreign Currency Gains (Losses) | (10.5) | (1.3)
Common equity securities, at fair value | 668.4 | 251.1
MediaAlpha |  |
Investment securities |  |
Cost or Amortized Cost | 0.0 | 0.0
Gross Unrealized Gains | 168.6 | 261.6
Gross Unrealized Losses | 0.0 | 0.0
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Common equity securities, at fair value | 168.6 | 261.6
Other long-term investments |  |
Investment securities |  |
Cost or Amortized Cost | 1,340.8 | 1,186.7
Gross Unrealized Gains | 271.1 | 239.0
Gross Unrealized Losses | (107.1) | (44.1)
Net Foreign Currency Gains (Losses) | (16.8) | (3.8)
Common equity securities, at fair value | $ 1,488.0 | $ 1,377.8

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