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Investment Securities - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions |  | 3 Months Ended |
--- | --- | --- | ---
Mar. 28, 2024 USD ($) $ / shares | Mar. 31, 2024 USD ($) fund $ / shares shares | Jun. 30, 2023 USD ($) $ / shares | Dec. 31, 2023 USD ($)
Concentration Risk [Line Items] |  |  |  |
Investments held in trusts |  | $ 589.6 |  | $ 585.6
Assets held by insurance regulators |  | 4.7 |  | 4.6
Other long-term investments |  | $ 1,991.9 |  | 1,998.2
MediaAlpha | IPO |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Increase in book value per share (in dollars per share) | $ / shares |  | $ 9.00 |  |
Fair value measured on a recurring basis | MediaAlpha |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments | $ 465.6 | $ 465.6 |  | 254.9
Lloyd’s trust deposits |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | 150.5 |  | 158.0
Lloyds Bank |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fair value investments |  | 345.6 |  | 344.7
Outrigger Re |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Short-term investments |  | 222.4 |  | $ 265.3
Bamboo Captive |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Short-term investments |  | $ 20.7 |  |
MediaAlpha |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Tender offer purchase price |  |  | $ 5.9 |
Purchase price (in dollars per share) | $ / shares | $ 20.37 |  | $ 10.00 |
Investment owned (in shares) | shares |  | 22.9 |  |
Ownership interest (as a percent) |  | 34.70% |  | 34.90%
Fully diluted ownership interest (as a percent) |  | 32.40% |  |
Increase in investment owned (in dollars per share) | $ / shares |  | $ 1.00 |  |
Largest Investment | Private Equity and Hedge Fund Concentration Risk | Investments |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | $ 59.7 |  | $ 57.3
HG Global Senior Notes | Senior Notes |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Interest reserve account |  | 30.5 |  | 30.4
Kudu Credit Facility | Secured Debt |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Restricted cash |  | 15.1 |  | 14.9
Asset Pledged as Collateral |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Short-term investments |  | 201.9 |  | 198.9
Asset Pledged as Collateral | Additional Investments, Insurance Regulators and Reinsurance Counterparties |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fair value investments |  | 204.6 |  | 244.5
Collateralized loan obligations |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fixed maturity investments, at fair value |  | 246.5 |  | 209.1
Collateralized loan obligations | Fair value measured on a recurring basis |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Fixed maturity investments, at fair value |  | 246.5 |  | 209.1
Private equity funds |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | $ 255.2 |  |
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 |  | $ 249.6 |  | 194.4
Foreign regulatory deposits |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | 150.5 |  | 158.0
ILS funds |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Other long-term investments |  | $ 109.7 |  | $ 160.5
ILS funds | Minimum |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Notice period |  | 30 days |  |
ILS funds | Maximum |  |  |  |
Concentration Risk [Line Items] |  |  |  |
Notice period |  | 90 days |  |

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