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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, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | $ 184.9 | $ 434.2 | $ (191.5)
Amount attributable to TPC Providers | 0.0 | 0.0 | 6.8
Investment in MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 38.0 | 27.1 | (93.0)
Net realized and unrealized investment (losses) gains |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 118.0 | 96.8 | 115.1
Net unrealized investment gains (losses) | 66.9 | 337.4 | (313.4)
Net realized and unrealized investment gains (losses) | 184.9 | 434.2 | (198.3)
Fixed maturity investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | (10.1) | (4.7) | (11.9)
Net unrealized investment gains (losses) | 3.9 | 66.1 | (168.4)
Short-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | (1.2) | (0.2) | (1.4)
Net unrealized investment gains (losses) | (0.8) | 1.9 | (1.1)
Common equity securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 3.5 | 29.5 | 0.0
Net unrealized investment gains (losses) | 58.1 | 47.1 | (7.1)
Net realized and unrealized investment gains (losses) | 61.6 | 76.6 | (7.1)
Equity Method Investments | Investment in MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 91.2 | 0.0 | 0.0
Net unrealized investment gains (losses) | (53.2) | 27.1 | (93.0)
Net realized and unrealized investment gains (losses) | 38.0 | 27.1 | (93.0)
Other long-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized investment gains (losses) | 34.6 | 72.2 | 128.4
Net unrealized investment gains (losses) | 58.9 | 195.2 | (43.8)
Fixed Maturities And Other Short-Term Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (8.2) | 63.1 | (182.8)
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 0.9 | 4.7 | (2.9)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (9.1) | 58.4 | (179.9)
Common Stock And Equity Method Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 99.6 | 103.7 | (100.1)
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 35.4 | 28.4 | 0.0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 64.2 | 75.3 | (100.1)
Total Investments After TPC Providers - Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Gain (loss) on investments, exchange rate effect | $ (31.5) | $ 13.6 | $ (29.3)

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