Income Taxes - Valuation Allowance (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Valuation Allowance [Line Items]
Valuation allowance	$ 116.1	$ 94.3
HG Global/BAM - Segment \| BAM
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	6.3	17.5
Net Operating Losses in Luxembourg Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	47.6	16.0
U S Losses and Other Federal Deferred Tax Benefits
Valuation Allowance [Line Items]
Valuation allowance	46.9	56.6
Net Operating Losses in Israel Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	20.8	20.4
Net Operating Losses in United Kingdom Subsidiaries
Valuation Allowance [Line Items]
Valuation allowance	0.8	1.3
Net Deferred Tax Assets For Certain U.S. Operations - Other Operations
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(7.2)	7.8
Net Deferred Tax Assets For BAM
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(8.7)	(10.9)
Investments and Net Operating Losses At United States Branches Of United Kingdom Holding Companies
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	(0.6)	4.0
Net Deferred Tax Assets For Certain U.S. Operations - Ark
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	0.1
Investments Held in Luxembourg-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	31.6	(9.2)
Investments Held in Israel-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	0.4	(1.5)
Investments Held in U.K.-Domiciled Subsidiaries
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance	$ (0.4)	$ 1.0

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-2

+ Details

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

- Definition

Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-2

+ Details

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

- 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

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=wtm_HGGlobalBAMSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsegmentsAxis=wtm_BAMSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_NetOperatingLossesInLuxembourgSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_USLossesAndOtherFederalDeferredTaxBenefitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_NetOperatingLossesinIsraelSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_NetOperatingLossesinUnitedKingdomSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_NetDeferredTaxAssetsForCertainU.S.OperationsOtherOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_NetDeferredTaxAssetsForBAMMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_InvestmentsAndNetOperatingLossesAtUnitedStatesBranchesOfUnitedKingdomHoldingCompaniesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_NetDeferredTaxAssetsForCertainU.S.OperationsArkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_InvestmentsHeldInLuxembourgDomiciledSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_InvestmentsHeldInIsraelDomiciledSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowanceByDeferredTaxAssetAxis=wtm_InvestmentsHeldInU.K.DomiciledSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: