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Investment Securities - Investments Held on Deposit or as Collateral (Details) - USD ($) $ in Millions | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |
Investments held in trusts | $ 517.5 | $ 500.5
Assets held by insurance regulators | 4.6 | 4.6
Other long-term investments | 1,643.0 | 1,488.0
Outrigger Re | Cash and Short Term Investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investment owned, balance | 206.0 | 203.7
HG Global Senior Notes | Secured Debt |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Interest reserve account | 31.6 | 31.2
Lloyd’s trust deposits |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 148.7 | 137.4
Additional Investments, Insurance Regulators and Reinsurance Counterparties | Asset Pledged as Collateral |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investments pledged as collateral | 263.3 | 257.0
Short-term investments | Asset Pledged as Collateral |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investments pledged as collateral | 144.1 | 90.3
Lloyds Bank |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | $ 326.3 | $ 319.2

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