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Investment Securities - Investment Holding Common Equities/ Other Investments/ MediaAlpha (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020
Common equity securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | $ 117.6 | $ 0.0
Gross Unrealized Gains | 1.1 | 0.0
Gross Unrealized Losses | (0.2) | 0.0
Net Foreign Currency Gains (Losses) | 0.1 | 0.0
Common equity securities, at fair value | 118.6 | 0.0
Marketing Technology (MediaAlpha) |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 0.0 | 0.0
Gross Unrealized Gains | 600.2 | 802.2
Gross Unrealized Losses | 0.0 | 0.0
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Common equity securities, at fair value | 600.2 | 802.2
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 965.5 | 767.4
Gross Unrealized Gains | 125.8 | 95.8
Gross Unrealized Losses | (56.5) | (78.1)
Net Foreign Currency Gains (Losses) | (2.6) | 1.7
Common equity securities, at fair value | $ 1,032.2 | $ 786.8

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