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Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) (1) | $ 226.8 | $ 198.1
MediaAlpha |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) (1) | 210.7 | 85.2
Net realized and unrealized investment (losses) gains |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | (5.7) | 46.7
Net unrealized investment gains (losses) | 232.5 | 151.4
Fixed maturity investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | (5.8) | (0.1)
Net unrealized investment gains (losses) | (7.4) | 29.7
Short-term investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | (0.2) | (0.4)
Net unrealized investment gains (losses) | (1.0) | 1.2
Common equity securities |  |
Gain (Loss) on Securities [Line Items] |  |
Net unrealized investment gains (losses) | 23.1 | 29.4
Net realized and unrealized investment gains (losses) (1) | 23.1 | 29.4
Equity Method Investments | MediaAlpha |  |
Gain (Loss) on Securities [Line Items] |  |
Net unrealized investment gains (losses) | 210.7 | 85.2
Net realized and unrealized investment gains (losses) (1) | 210.7 | 85.2
Other long-term investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | 0.3 | 47.2
Net unrealized investment gains (losses) | 7.1 | 5.9
Fixed maturity and short-term investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) (1) | (14.4) | 30.4
Less: net realized and unrealized gains (losses) on investment securities sold during the period | (0.7) | 1.8
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (13.7) | 28.6
Common equity securities and investment in MediaAlpha |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) (1) | 233.8 | 114.6
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 0.0 | 0.0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 233.8 | 114.6
Total Investments After TPC Providers - Investments |  |
Gain (Loss) on Securities [Line Items] |  |
Gain (loss) on investments, exchange rate effect | $ (11.5) | $ 5.4

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