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Investments - Realized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | $ 4.9 | $ 3.2 | $ (4.1) | $ (15.3)
Fixed maturity securities |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | 1.5 | (0.7) | (3.9) | 0.4
Equity securities |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | 4.4 | 7.1 | 1.7 | (7.6)
Short-term investments and other |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | $ (1.0) | $ (3.2) | $ (1.9) | $ (8.1)

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- Definition Amount of realized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 + 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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