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Derivatives (Details) - White Mountains Life Re ¥ in Billions, $ in Billions	Dec. 31, 2015 USD ($)	Dec. 31, 2015 JPY (¥)
Derivative [Line Items]
Notional amount	$ 0.4	¥ 50.7
Collective account values of underlying variable annuities as a percentage of the guarantee value	109.00%	109.00%

X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
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

No definition available.

+ Details

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

X

- Definition

Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability.

+ References

No definition available.

+ Details

Name:	wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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