Percentage investment held by members or limited partners of limited liability company (LLC) or limited partnership (LP).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482987/272-10-50-3

+ Details

Name:	us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash outflow related to the acquisition or continued holding of limited partnership interests held for investment purposes.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-13

+ Details

Name:	us-gaap_PaymentsToAcquireLimitedPartnershipInterests
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Limited Liability Company or Limited Partnership, Geographical Presence

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest, Look-through Basis

+ References

No definition available.

+ Details

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

- Definition

Other Significant Transactions [Line Items]

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=wtm_BroadStreetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=wtm_BroadStreetMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	dei_LegalEntityAxis=wtm_BishopStreetMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Significant Transactions - BroadStreet and Bishop Street (Details) $ in Millions	Feb. 26, 2026 USD ($)	Jul. 18, 2025 USD ($) country
United States
Other Significant Transactions [Line Items]
Geographical presence \| country		50
Canadian
Other Significant Transactions [Line Items]
Geographical presence \| country		10
BroadStreet
Other Significant Transactions [Line Items]
Limited liability company or limited partnership, members or limited partners, ownership interest		10.90%
Equity interest percent of ownership basis		5.00%
BroadStreet
Other Significant Transactions [Line Items]
Payments to acquire limited partnership interests \| $		$ 150.0
Bishop Street
Other Significant Transactions [Line Items]
Payments to acquire limited partnership interests \| $	$ 125.0