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

Fair value of financial and nonfinancial obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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

Represents the foreign exchange volatilities, used as an input to measure fair value.

No definition available.

Name:	wtm_FairValueInputsForeignExchangeVolatilities
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the index volatilities, used as an input to measure fair value.

No definition available.

Name:	wtm_FairValueInputsIndexVolatilities
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the mortality, used as an input to measure fair value.

No definition available.

Name:	wtm_FairValueInputsMortality
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Significant unobservable inputs for equity index options

No definition available.

Name:	wtm_FairValueInputsSignificantUnobservableInputsForEquityIndexOptions
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the surrenders, used as an input to measure fair value.

No definition available.

Name:	wtm_FairValueInputsSurrenders
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

Name:	us-gaap_ValuationTechniqueAxis=wtm_DiscountedCashFlowValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_VariableAnnuityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_ForeignExchangeOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_ValuationTechniqueAxis=wtm_CounterpartyvaluationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FairValueByLiabilityClassAxis=wtm_EquityIndexOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: