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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | $ (287.3) | $ (198.4) | $ (159.2)
Investment expenses | (11.6) | (10.3) | (11.7)
Net investment income | 275.7 | 188.1 | 147.5
Fixed income securities |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (115.7) | (95.6) | (87.2)
Short-term investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (5.3) | (0.6) | (0.1)
Short-term investment, Trading |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (0.3) | (0.1) | 0.0
Fixed term deposits (included in cash and cash equivalents) |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (39.9) | (6.6) | (0.7)
Catastrophe bonds — Trading |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (0.2) | (0.3) | (0.9)
Privately-held investments, Available for Sale |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (0.1) | 0.0 | 0.0
Privately-held investments — Trading |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (44.7) | (24.3) | (18.2)
Other investments, at fair value |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (17.8) | (13.9) | (21.9)
Fixed Income Securities |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | $ (98.9) | $ (57.0) | $ (30.2)

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- Definition Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 45 -Paragraph 39 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480228/946-830-45-39 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.1-5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1 + Details
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- Definition Amount of expenses related to the generation of investment income. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 55 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480167/946-830-55-10 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)(c)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2 + Details
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- Definition Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 55 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480167/946-830-55-10 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)(d)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-07(4)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483575/946-220-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section 45 -Paragraph 5 -SubTopic 220 -Topic 946 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483581/946-220-45-5 + Details
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