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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2024 USD ($) $ / shares shares | Jun. 30, 2023 $ / shares shares | Dec. 31, 2024 USD ($) fund $ / shares shares | Dec. 31, 2023 USD ($) $ / shares | Dec. 31, 2022 USD ($)
Concentration Risk [Line Items] |  |  |  |  |
Gross realized investment gains |  |  | $ 139.1 | $ 113.7 | $ 129.9
Loss on sale of investment securities |  |  | 21.1 | 16.9 | 14.8
Proceeds from sale and maturity of investments |  |  | $ 1,200.0 | $ 800.0 | $ 500.0
Weighted average portfolio period |  |  | 1 year 10 months 24 days | 1 year 10 months 24 days |
Weighted average portfolio period, excluding short-term invetments |  |  | 2 years 7 months 6 days | 3 years 3 months 18 days |
Other long-term investments |  |  | $ 2,150.2 | $ 1,998.2 |
Total investments |  |  | 6,477.6 | 6,388.7 |
Level 3 | Recent transaction |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | 23.4 | 19.0 |
Private equity funds |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | $ 249.5 |  |
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 |  |
Bank loan fund |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | $ 264.7 | 194.4 |
Other long-term investments, redemption, written notice period |  |  | 15 days |  |
Foreign Regulatory Deposits |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | $ 149.9 | 158.0 |
ILS funds |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | $ 74.0 | 160.5 |
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 |  |  | $ 236.7 | 209.1 |
Collateralized loan obligations | Level 3 |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Fixed maturity investments, at fair value |  |  | 0.0 | 0.0 |
Fair value measured on a recurring basis | MediaAlpha |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | 201.6 | 254.9 |
Fair value measured on a recurring basis | MediaAlpha | Level 3 |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | 0.0 | 0.0 |
Fair value measured on a recurring basis | Collateralized loan obligations |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Fixed maturity investments, at fair value |  |  | 236.7 | 209.1 |
Fair value measured on a recurring basis | Collateralized loan obligations | Level 3 |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Fixed maturity investments, at fair value |  |  | 0.0 | 0.0 |
Lloyd’s trust deposits |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Other long-term investments |  |  | 149.9 | 158.0 |
Lloyds Bank |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Total investments |  |  | 361.5 | 345.2 |
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Total investments |  |  | 226.5 | 244.5 |
Outrigger Re |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Short-term investments, at fair value |  |  | 203.7 | $ 265.3 |
Bamboo CRV |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Restricted cash |  |  | 32.2 |  |
Pledged collateral |  |  | $ 46.7 |  |
MediaAlpha |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Shares purchased (in shares) | shares | 7.6 | 5.9 |  |  |
Purchase price (in dollars per share) | $ / shares | $ 19.00 | $ 10.00 | $ 11.29 | $ 11.15 |
Net of underwriting fees (in dollars per share) | $ / shares | $ 18.24 |  |  |  |
Investment owned (in shares) | shares |  |  | 17.9 |  |
Ownership interest (as a percent) |  |  | 26.60% | 34.90% |
Fully diluted ownership interest (as a percent) |  |  | 25.40% |  |
Increase in investment owned (in dollars per share) | $ / shares |  |  | $ 1.00 |  |
MediaAlpha | Secondary Offering |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |
Number of shares issued (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 |  |  | $ 59.2 | $ 57.3 |

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