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Investments - Other Investments (Details) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2023 | Dec. 31, 2022
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Opening undistributed value of investment, beginning balance |  | $ 6.2 | $ 3.9
Investment in the period |  | 0.4 | 2.0
Proceeds from Sale of Equity Method Investments |  | 0.0 | 0.0
Closing value of investment, ending balance |  | 7.6 | 6.2
Fair Value, Recurring [Member] |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Investments, fair value |  | 117.1 | 134.6
Equity Method Investment, Nonconsolidated Investee, Other |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Unrealized Gain (Loss) on Investments |  | 1.0 | 0.3
MVI |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Opening undistributed value of investment, beginning balance |  | 0.8 | 0.5
Investment in the period | $ 0.8 | 0.0 | 0.4
Proceeds from Sale of Equity Method Investments |  | 0.0 | 0.0
Closing value of investment, ending balance |  | 1.0 | 0.8
MVI | Equity Method Investment, Nonconsolidated Investee, Other |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Unrealized Gain (Loss) on Investments |  | 0.2 | (0.1)
Multi-Line Insurer |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Opening undistributed value of investment, beginning balance |  | 5.2 | 3.2
Investment in the period |  | 0.4 | 1.6
Proceeds from Sale of Equity Method Investments |  | 0.0 | 0.0
Closing value of investment, ending balance |  | 6.4 | 5.2
Multi-Line Insurer | Equity Method Investment, Nonconsolidated Investee, Other |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Unrealized Gain (Loss) on Investments |  | 0.8 | 0.4
Digital Re |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Opening undistributed value of investment, beginning balance |  | 0.2 | 0.2
Investment in the period |  | 0.0 | 0.0
Proceeds from Sale of Equity Method Investments |  | 0.0 | 0.0
Closing value of investment, ending balance |  | 0.2 | 0.2
Digital Re | Equity Method Investment, Nonconsolidated Investee, Other |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |
Unrealized Gain (Loss) on Investments |  | $ 0.0 | $ 0.0

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- Definition Equity Method Investment, Aggregate Cost [Roll Forward] + References No definition available. + Details
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- Definition This element represents the aggregate cost of investments accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 + Details
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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition The cash inflow associated with the sale of equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-12 + Details
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- Definition Amount of unrealized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-28 + Details
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