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Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | $ (40.6) | $ (156.7)
Less: net losses on investment securities sold during the period | (0.2) | (8.7)
MediaAlpha |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | (41.7) | 30.0
Investments Held |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | (40.4) | (148.0)
Fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | (25.4) | 2.2
Short-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | (1.3) | 0.4
Common equity securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | 1.0 | (140.8)
MediaAlpha |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | (41.7) | 30.0
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | 28.1 | (48.5)
Gain (loss) on investments, exchange rate effect | (0.2) | (0.4)
Amount attributable to TPC Providers |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha | $ (1.3) | $ 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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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