XML 130 R105.htm IDEA: XBRL DOCUMENT v3.25.4

Municipal Bond Guarantee Reinsurance - Schedule of Municipal Bond Guarantee Insured Obligations (Details) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) contract	Dec. 31, 2024 USD ($) contract
Subsidiary - HG Global | Maximum
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%
HG Global
Guarantor Obligations [Line Items]
Contracts outstanding | contract	17,377	15,884
Remaining weighted average contract period(in years)	11 years 7 months 6 days	11 years 2 months 12 days
Outstanding par value of policies assumed (in millions)	$ 20,559.9	$ 18,503.3
Gross unearned insurance premiums (in millions)	$ 327.9	$ 297.3

X

- Definition

Amount of unearned premium revenue under contracts issued by an insurance entity that provide protection to the holder of a financial obligation from a financial loss in the event of a default.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-4

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-4

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-4

+ Details

Name:	us-gaap_GuaranteeObligationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Municipal bond contracts issued and outstanding.

+ References

No definition available.

+ Details

Name:	wtm_FinancialGuaranteeInsuranceContractsNumberOfContracts
Namespace Prefix:	wtm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Financial Guarantee Insurance Contracts, Principal Balance

+ References

No definition available.

+ Details

Name:	wtm_FinancialGuaranteeInsuranceContractsPrincipalBalance
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Financial Guarantee Insurance Contracts, Remaining Weighted Average Contract Period

+ References

No definition available.

+ Details

Name:	wtm_FinancialGuaranteeInsuranceContractsRemainingWeightedAverageContractPeriod
Namespace Prefix:	wtm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of par value of policy reinsured.

+ References

No definition available.

+ Details

Name:	wtm_PercentageOfParValueOfPolicyReinsured
Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	srt_ConsolidatedEntitiesAxis=wtm_SubsidiaryHGGlobalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=wtm_HGGlobalSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: