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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2023 USD ($) $ / shares | Dec. 31, 2023 USD ($) fund $ / shares shares | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Concentration Risk [Line Items] |  |  |  |
Gross realized investment gains |  | $ 113.7 | $ 129.9 | $ 212.3
Gross realized investment losses |  | (16.9) | (14.8) | (55.5)
Proceeds from sale and maturity of investments |  | $ 800.0 | 500.0 | $ 800.0
Weighted average portfolio period |  | 1 year 10 months 24 days |  |
Weighted average portfolio period, excluding short-term invetments |  | 3 years 3 months 18 days |  |
Investments held in trusts |  | $ 585.6 | 500.5 |
Assets held by insurance regulators |  | 4.6 | 4.6 |
Other long-term investments |  | 1,998.2 | 1,488.0 |
Fair value investments |  | 6,388.7 | 5,170.0 |
Private equity funds |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | $ 248.6 |  |
Number of investments | fund |  | 16 |  |
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 |  | $ 194.4 | 174.8 |
Other long-term investments, redemption, written notice period |  | 15 days |  |
Foreign Regulatory Deposits |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | $ 158.0 | 137.4 |
ILS funds |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | $ 160.5 | 49.3 |
ILS funds | Minimum |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Notice period |  | 30 days |  |
MediaAlpha | IPO |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Increase in book value per share (in dollars per share) | $ / shares |  | $ 9.00 |  |
Collateralized loan obligations |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fixed maturity investments, at fair value |  | $ 209.1 | 182.9 |
Fair value measured on a recurring basis |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fair value investments |  | 6,388.7 | 5,170.0 |
Fair value measured on a recurring basis | MediaAlpha |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | 254.9 | 168.6 |
Fair value measured on a recurring basis | Collateralized loan obligations |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fixed maturity investments, at fair value |  | 209.1 | 182.9 |
Asset Pledged as Collateral |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Short-term investments |  | 198.9 | 90.3 |
Lloyd’s trust deposits |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | 158.0 | 137.4 |
Lloyds Bank |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fair value investments |  | 344.7 | 319.2 |
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fair value investments |  | 244.5 | 257.0 |
Outrigger Re |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Short-term investments |  | $ 265.3 | $ 203.7 |
MediaAlpha |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Tender offer purchase price | $ 5.9 |  |  |
Purchase price (in dollars per share) | $ / shares | $ 10.00 | $ 11.15 |  |
Investment owned (in shares) | shares |  | 22.9 |  |
Ownership interest (as a percent) |  | 34.90% | 27.10% |
Fully diluted ownership interest (as a percent) |  | 33.10% |  |
Increase in investment owned (in dollars per share) | $ / shares |  | $ 1.00 |  |
Largest Investment | Investments | Private Equity and Hedge Fund Concentration Risk |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | $ 57.3 | $ 49.0 |
HG Global Senior Notes | Senior Notes |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Interest reserve account |  | 30.4 | 31.2 |
Kudu Credit Facility | Secured Debt |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Interest reserve account |  | $ 14.9 |  |
Restricted cash |  |  | $ 12.2 |

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