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Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | $ 434.2 | $ (191.5) | $ (246.1)
Amount attributable to TPC Providers | 0.0 | 6.8 | (7.7)
Investment in MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 27.1 | (93.0) | (380.3)
Net realized and unrealized investment (losses) gains |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 96.8 | 115.1 | 156.8
Net unrealized investment gains (losses) | 337.4 | (313.4) | (395.2)
Net realized and unrealized investment gains (losses) | 434.2 | (198.3) | (238.4)
Fixed maturity investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | (4.7) | (11.9) | 3.9
Net unrealized investment gains (losses) | 66.1 | (168.4) | (42.2)
Short-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | (0.2) | (1.4) | (0.1)
Net unrealized investment gains (losses) | 1.9 | (1.1) | 0.0
Common equity securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 29.5 | 0.0 | 0.4
Net unrealized investment gains (losses) | 47.1 | (7.1) | 14.9
Net realized and unrealized investment gains (losses) | 76.6 | (7.1) | 15.3
Equity Method Investments | Investment in MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 0.0 | 0.0 | 160.3
Net unrealized investment gains (losses) | 27.1 | (93.0) | (540.6)
Net realized and unrealized investment gains (losses) | 27.1 | (93.0) | (380.3)
Other long-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 72.2 | 128.4 | (7.7)
Net unrealized investment gains (losses) | 195.2 | (43.8) | 172.7
Fixed Maturities And Other Short-Term Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 63.1 | (182.8) | (38.4)
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 4.7 | (2.9) | (8.4)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 58.4 | (179.9) | (30.0)
Common Stock And Equity Method Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 103.7 | (100.1) | (365.0)
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 28.4 | 0.0 | 20.3
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 75.3 | (100.1) | (385.3)
Total Investments After TPC Providers - Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Gain (loss) on investments, exchange rate effect | $ 13.6 | $ (29.3) | $ (7.7)

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