XML 151 R134.htm IDEA: XBRL DOCUMENT
v3.25.0.1

SCHEDULE VI - SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Current Year | $ 914.4 | $ 706.9 | $ 588.1
Prior Year | (58.6) | 19.9 | (51.7)
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Deferred acquisition costs | 165.2 | 145.3 | 127.2
Reserves for Unpaid Claims and Claims Adjustment Expenses | 2,127.5 | 1,605.1 | 1,296.5
Discount, if any, deducted in Column C | 0.0 | 0.0 | 0.0
Unearned Premiums | 853.3 | 743.6 | 623.2
Earned Premiums | 1,587.8 | 1,409.7 | 1,043.4
Net investment income | 90.7 | 61.4 | 16.3
Current Year | 914.4 | 706.9 | 588.1
Prior Year | (58.6) | 19.9 | (51.7)
Amortization of deferred policy acquisition costs | 294.1 | 273.1 | 238.3
Paid Claims and Claims Adjustment Expenses | 416.7 | 407.9 | 257.5
Net Premiums written | 1,679.1 | 1,520.9 | 1,195.2
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | Bamboo Captive |  |  |
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Deferred acquisition costs | 10.2 | 0.0 | 0.0
Reserves for Unpaid Claims and Claims Adjustment Expenses | 17.8 | 0.0 | 0.0
Discount, if any, deducted in Column C | 0.0 | 0.0 | 0.0
Unearned Premiums | 31.5 | 0.0 | 0.0
Earned Premiums | 39.4 | 0.0 | 0.0
Net investment income | 1.4 | 0.0 | 0.0
Current Year | 17.3 | 0.0 | 0.0
Prior Year | 3.3 | 0.0 | 0.0
Amortization of deferred policy acquisition costs | 14.1 | 0.0 | 0.0
Paid Claims and Claims Adjustment Expenses | 12.0 | 0.0 | 0.0
Net Premiums written | 50.0 | 0.0 | 0.0
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity | Bamboo CRV |  |  |
SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS |  |  |
Deferred acquisition costs | 5.8 | 0.0 | 0.0
Reserves for Unpaid Claims and Claims Adjustment Expenses | 12.1 | 0.0 | 0.0
Discount, if any, deducted in Column C | 0.0 | 0.0 | 0.0
Unearned Premiums | 29.0 | 0.0 | 0.0
Earned Premiums | 32.7 | 0.0 | 0.0
Net investment income | 0.4 | 0.0 | 0.0
Current Year | 12.1 | 0.0 | 0.0
Prior Year | 0.0 | 0.0 | 0.0
Amortization of deferred policy acquisition costs | 12.1 | 0.0 | 0.0
Paid Claims and Claims Adjustment Expenses | 0.0 | 0.0 | 0.0
Net Premiums written | $ 61.7 | $ 0.0 | $ 0.0

X
- 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column A -Subparagraph (c) -Publisher SEC Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column A -Subparagraph (b) -Publisher SEC Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column A -Subparagraph (a) -Publisher SEC Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column B -Publisher SEC Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column G -Publisher SEC Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column H -Publisher SEC Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column I -Publisher SEC Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column J -Publisher SEC Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column K -Publisher SEC Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column F -Publisher SEC Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column E -Publisher SEC Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column D -Publisher SEC Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 18 -Paragraph Column C -Publisher SEC Reference 14: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column I)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 15: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column B)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 16: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column D)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 17: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column F)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 18: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column G)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 19: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column J)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 20: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column K)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 21: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column C)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 22: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column E)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 23: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column H)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 24: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column A)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 25: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column A)(b)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 Reference 26: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column A)(c)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | srt_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersLineItems
Namespace Prefix: | srt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of amortization expense on deferred policy acquisition cost for property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column I)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of deferred policy acquisition cost related to property-casualty insurance policy written. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column B)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of discount deducted from reserve for unpaid claim and claim adjustment expense by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column D)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDiscountDeductedFromReserves
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of premium revenue earned by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column F)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of net investment income earned by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column G)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of loss reported and paid for claim and claim adjustment expense by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column J)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of premium written by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column K)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of reserve, policy claim payable and loss expense incurred by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column C)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of premium written but not yet earned by property-casualty insurance underwriter. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18(Column E)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-3 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_ArkInsuranceHoldingsLimitedWMOutriggerReSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersByAffiliationWithRegistrantAxis=srt_ConsolidatedPropertyAndCasualtyInsuranceEntityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_BambooCaptiveSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_BambooCRVMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |