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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, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | $ 172.6 | $ 152.9 | $ 132.0 | $ (3.8)
Less: net (losses) gains on investment securities sold during the period | (0.1) | 2.2 | (0.3) | (6.5)
Investments Held |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | 172.7 | 150.7 | 132.3 | 2.7
Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | 9.0 | 30.0 | (16.4) | 32.2
Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | 1.6 | 0.0 | 0.3 | 0.4
Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | 5.1 | 98.3 | 6.1 | (42.5)
Marketing Technology (MediaAlpha) |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | 113.0 | 15.0 | 71.3 | 45.0
Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | 47.4 | 9.6 | 75.5 | (38.9)
Gain (loss) on investments, exchange rate effect | (0.2) | 0.4 | (0.4) | 0.0
Amount attributable to TPC Providers |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) | $ (3.5) | $ 0.0 | $ (4.8) | $ 0.0

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- Definition Amount of realized and unrealized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27405-111563 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
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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 -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
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