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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, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Gain (Loss) on Securities [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | $ 361.5 | $ 184.9 | $ 434.2
MediaAlpha |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 29.6 | 38.0 | 27.1
Net realized and unrealized investment (losses) gains |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized investment gains (losses) | 111.5 | 118.0 | 96.8
Net unrealized investment gains (losses) | 250.0 | 66.9 | 337.4
Fixed maturity investments |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized investment gains (losses) | 0.8 | (10.1) | (4.7)
Net unrealized investment gains (losses) | 53.9 | 3.9 | 66.1
Short-term investments |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized investment gains (losses) | 0.5 | (1.2) | (0.2)
Net unrealized investment gains (losses) | 0.5 | (0.8) | 1.9
Common equity securities |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized investment gains (losses) | 67.7 | 3.5 | 29.5
Net unrealized investment gains (losses) | (14.2) | 58.1 | 47.1
Net realized and unrealized investment gains (losses) | 53.5 | 61.6 | 76.6
Investment in MediaAlpha | MediaAlpha |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized investment gains (losses) | 0.0 | 91.2 | 0.0
Net unrealized investment gains (losses) | 29.6 | (53.2) | 27.1
Net realized and unrealized investment gains (losses) | 29.6 | 38.0 | 27.1
Other long-term investments |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized investment gains (losses) | 42.5 | 34.6 | 72.2
Net unrealized investment gains (losses) | 180.2 | 58.9 | 195.2
Fixed maturity and short-term investments |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 55.7 | (8.2) | 63.1
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 5.7 | 0.9 | 4.7
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 50.0 | (9.1) | 58.4
Common equity securities and investment in MediaAlpha |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 83.1 | 99.6 | 103.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 6.4 | 35.4 | 28.4
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 76.7 | 64.2 | 75.3
Total Investments IncludingTPC Providers - Investments |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Gain (loss) on investments, exchange rate effect | $ 37.1 | $ (31.5) | $ 13.6

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