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Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains | $ 306.6 | $ 67.0 | $ 302.8 | $ 347.8
Less: net gains on investment securities sold during the period | 36.1 | 0.0 | 29.6 | 12.4
Investments Held |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains | 270.5 | 67.0 | 273.2 | 335.4
Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains | 3.6 | 8.9 | 35.8 | 46.9
Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains | 0.0 | 0.0 | 0.4 | 0.3
Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains | 48.4 | 2.4 | 5.9 | 141.5
Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains | 254.6 | 55.7 | 260.7 | 159.1
Gain (loss) on investments, exchange rate effect | 0.8 | (0.8) | 0.8 | (0.8)
Other long-term investments | MediaAlpha |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Realized gain associated with unconsolidated affiliates |  |  |  | 67.5
Unrealized gain associated with unconsolidated affiliates |  |  |  | 114.7
MediaAlpha | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains | $ 250.0 | $ 35.3 | $ 295.0 | $ 150.0

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