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Investment Securities - Investments Held on Deposit or as Collateral (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021
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Debt and Equity Securities, FV-NI [Line Items] |  |
Investments held in trusts | $ 463.7 | $ 479.5
Assets held by insurance regulators | 4.8 | 4.8
Other long-term investments | 1,428.5 | 1,377.8
Lloyd’s trust deposits |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other long-term investments | 113.2 | 113.8
Short-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investments pledged as collateral | 50.0 | 50.0
Lloyds Bank |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investments held in trusts | 113.2 |
Fair value investments | $ 305.6 | $ 342.8

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