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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2025 USD ($) fund $ / shares shares | Jun. 30, 2024 USD ($) $ / shares shares | Dec. 31, 2024 USD ($) $ / shares
Concentration Risk [Line Items] |  |  |
Other long-term investments | $ 2,287.9 |  | $ 2,150.2
Total investments | 6,551.3 |  | 6,477.6
Private equity funds |  |  |
Concentration Risk [Line Items] |  |  |
Other long-term investments | $ 240.6 |  |
Number of investments | fund | 17 |  |
Hedge funds |  |  |
Concentration Risk [Line Items] |  |  |
Number of investments | fund | 2 |  |
Hedge funds | Minimum |  |  |
Concentration Risk [Line Items] |  |  |
Notice period | 45 days |  |
Hedge funds | Maximum |  |  |
Concentration Risk [Line Items] |  |  |
Notice period | 90 days |  |
Bank loan fund |  |  |
Concentration Risk [Line Items] |  |  |
Other long-term investments | $ 264.9 |  | 264.7
Written notice period | 15 days |  |
Lloyd’s trust deposits |  |  |
Concentration Risk [Line Items] |  |  |
Other long-term investments | $ 155.9 |  | 149.9
ILS funds |  |  |
Concentration Risk [Line Items] |  |  |
Other long-term investments | $ 71.3 |  | 74.0
ILS funds | Minimum |  |  |
Concentration Risk [Line Items] |  |  |
Notice period | 30 days |  |
ILS funds | Maximum |  |  |
Concentration Risk [Line Items] |  |  |
Notice period | 90 days |  |
IPO | MediaAlpha |  |  |
Concentration Risk [Line Items] |  |  |
Increase in book value per share (in dollars per share) | $ / shares | $ 7.00 |  |
Collateralized loan obligations |  |  |
Concentration Risk [Line Items] |  |  |
Fixed maturity investments, at fair value | $ 256.0 |  | 236.7
Fair value measured on a recurring basis | MediaAlpha |  |  |
Concentration Risk [Line Items] |  |  |
Other long-term investments | 165.0 |  | 201.6
Fair value measured on a recurring basis | Collateralized loan obligations |  |  |
Concentration Risk [Line Items] |  |  |
Fixed maturity investments, at fair value | 256.0 |  | 236.7
Lloyd’s trust deposits |  |  |
Concentration Risk [Line Items] |  |  |
Other long-term investments | 155.9 |  | 149.9
Lloyds Bank |  |  |
Concentration Risk [Line Items] |  |  |
Total investments | 348.2 |  | 361.5
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral |  |  |
Concentration Risk [Line Items] |  |  |
Total investments | 250.3 |  | 226.5
Outrigger Re |  |  |
Concentration Risk [Line Items] |  |  |
Short-term investments, at fair value | 208.7 |  | 203.7
Bamboo CRV |  |  |
Concentration Risk [Line Items] |  |  |
Restricted cash | 41.3 |  | 32.2
Pledged collateral | $ 42.7 |  | $ 46.7
MediaAlpha |  |  |
Concentration Risk [Line Items] |  |  |
Shares purchased (in shares) | shares |  | 7.6 |
Purchase price (in dollars per share) | $ / shares | $ 9.24 | $ 19.00 | $ 11.29
Purchase price net of underwriting fees (in dollars per share) | $ / shares |  | $ 18.24 |
Investment owned (in shares) | shares | 17.9 |  |
Ownership interest (as a percent) | 26.50% |  | 26.60%
Fully diluted ownership interest (as a percent) | 24.30% |  |
Increase in investment owned (in dollars per share) | $ / shares | $ 1.00 |  |
MediaAlpha | Secondary Offering |  |  |
Concentration Risk [Line Items] |  |  |
Sale of stock, number of shares issued in transaction (in shares) | shares |  | 5.0 |
Proceeds from sale of business |  | $ 91.2 |
Largest Investment | Investments | Private Equity and Hedge Fund Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Other long-term investments | $ 80.6 |  | $ 59.2

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