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Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized investment gains (losses) | $ 12.6 | $ (2.9) | $ 59.4 | $ 14.5
Net unrealized investment gains (losses) | (1.2) | (217.4) | 150.1 | (226.5)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (77.3) | (113.5) | 7.9 | (94.7)
Gain (loss) on investments, exchange rate effect | 5.5 | (24.5) | 10.9 | (21.8)
Investments Held |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | 11.4 | (217.6) | 209.5 | (207.2)
Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized investment gains (losses) | (1.3) | (4.8) | (1.4) | (5.9)
Net unrealized investment gains (losses) | (11.5) | (54.6) | 18.2 | (134.1)
Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized investment gains (losses) | 0.2 | (0.1) | (0.1) | (0.3)
Net unrealized investment gains (losses) | 0.2 | (0.7) | 1.3 | (0.5)
Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized investment gains (losses) | 12.6 | 0.0 | 12.6 | 0.0
Net unrealized investment gains (losses) | 18.9 | (11.6) | 48.3 | (10.6)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 31.5 | (11.6) | 60.9 | (10.6)
Marketing Technology (MediaAlpha) |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net unrealized investment gains (losses) | (77.3) | (113.5) | 7.9 | (94.7)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (77.3) | (113.5) | 7.9 | (94.7)
Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized investment gains (losses) | 1.1 | 2.0 | 48.3 | 20.7
Net unrealized investment gains (losses) | 68.5 | (37.0) | 74.4 | 13.4
Net realized and unrealized investment (losses) gains |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 11.4 | (220.3) | 209.5 | (212.0)
TPC Providers |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 0.0 | 2.7 | 0.0 | 4.8
Fixed maturity and short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (12.4) | (60.2) | 18.0 | (140.8)
Net realized and unrealized investment gains (losses) | 0.4 | (2.1) | 2.2 | (3.1)
Fixed maturity and short-term investments | Investments Held |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (12.8) | (58.1) | 15.8 | (137.7)
Common equity securities and investment in MediaAlpha |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (45.8) | (125.1) | 68.8 | (105.3)
Net realized and unrealized investment gains (losses) | 3.8 | 0.0 | 3.8 | 0.0
Common equity securities and investment in MediaAlpha | Investments Held |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | $ (49.6) | $ (125.1) | $ 65.0 | $ (105.3)

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- Definition The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-28 + Details
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