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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | $ (159.2) | $ (167.3) | $ (209.1)
Investment expenses | (11.7) | (12.7) | (11.8)
Net investment income | 147.5 | 154.6 | 197.3
Fixed income securities |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (87.2) | (106.5) | (128.2)
Short-term investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (0.1) | (0.8) | (2.3)
Short-term investment, Trading |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | 0.0 | (0.5) | (2.5)
Fixed term deposits (included in cash and cash equivalents) |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (0.7) | (6.4) | (19.5)
Catastrophe bonds — Trading |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (0.9) | (1.4) | (2.3)
Privately-held investments — Trading |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (18.2) | (20.9) | (3.4)
Other investments, at fair value |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | (21.9) | (2.0) | (8.9)
Fixed Income Securities |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Investment income | $ (30.2) | $ (32.8) | $ (42.0)

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.1-5) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 + Details
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- Definition Amount of expenses related to the generation of investment income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 + 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(2)) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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