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Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021
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Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Total Reinsurance recoverables on unpaid losses | $ 505.0 | $ 428.9
Ark |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 791.5 | 465.8
Total Reinsurance recoverables on unpaid losses | 505.0 | 428.9
Loss and loss adjustment expense reserves | 1,296.5 | 894.7
Ark | Property and Accident & Health |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Loss and LAE reserves, net of reinsurance | 258.2 | 175.0
Total Reinsurance recoverables on unpaid losses | 224.6 | 145.2
Ark | Specialty |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Loss and LAE reserves, net of reinsurance | 204.3 | 85.2
Total Reinsurance recoverables on unpaid losses | 97.2 | 68.9
Ark | Marine & Energy |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Loss and LAE reserves, net of reinsurance | 196.4 | 99.3
Total Reinsurance recoverables on unpaid losses | 79.8 | 70.2
Ark | Casualty - Active |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Loss and LAE reserves, net of reinsurance | 71.5 | 37.4
Total Reinsurance recoverables on unpaid losses | 49.9 | 41.4
Ark | Casualty - Runoff |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Loss and LAE reserves, net of reinsurance | 60.8 | 68.4
Total Reinsurance recoverables on unpaid losses | 53.5 | 103.2
Ark | Other |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |
Loss and LAE reserves, net of reinsurance | $ 0.3 | $ 0.5

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(13)(a)) -URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Definition Amount before valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, and for amounts expected based upon statistical projections and other measures of incurred losses and loss settlement expenses which have not yet been reported to the ceding insurer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI https://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI https://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489 + Details
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