Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Dec. 31, 2015 USD ($) Rating
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	$ 20,100
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	$ 19,000
S&P Credit Ratings \| Rating	23
Derivative, Collateral, Right to Reclaim Cash	$ 0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	20,100
Excess collateral provided to counterparty in cash	5,800
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	6,900
A \| Citigroup OTC [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	9,900
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	8,500
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	9,900
Excess collateral provided to counterparty in cash	0
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	1,400
A \| Bank of America [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	700
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	700
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	700
Excess collateral provided to counterparty in cash	0
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	0
A \| Citigroup Exchange Traded [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	1,000
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	6,800
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1,000
Excess collateral provided to counterparty in cash	5,800
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	0
Standard & Poor's, A+ Rating [Member] \| J P Morgan Chase [Member]
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	8,500
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	3,000
Derivative, Collateral, Right to Reclaim Cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	8,500
Excess collateral provided to counterparty in cash	0
Excess collateral providedto counterparty in financial instruments	0
Derivative, Collateral, Obligation to Return Cash	$ 5,500

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226016-175313

+ Details

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

- Definition

Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226016-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=d3e41678-113959

+ Details

Name:	us-gaap_DerivativeCollateralRightToReclaimCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of right to receive securities collateral under master netting arrangements that have not been offset against derivative liabilities.

+ References

+ Details

Name:	us-gaap_DerivativeCollateralRightToReclaimSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

+ References

+ Details

Name:	us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after effects of master netting arrangements, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset offset against an obligation to return collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226016-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Excess collateral provided to counterparty in cash.

+ References

No definition available.

+ Details

Name:	wtm_Excesscollateralprovidedtocounterpartyincash
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Excess collateral provided to counterparty in financial instruments

+ References

No definition available.

+ Details

Name:	wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that were not covered by collateral.

+ References

No definition available.

+ Details

Name:	wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the S&P creditworthiness heirarchy ratings used by the entity.

+ References

No definition available.

+ Details

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

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

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

- Details

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

- Details

Name:	us-gaap_CreditRatingStandardPoorsAxis=us-gaap_StandardPoorsARatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=wtm_CitigroupOtcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=wtm_BankOfAmericaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=wtm_CitigroupExchangeTradedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditRatingStandardPoorsAxis=us-gaap_StandardPoorsAPlusRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=wtm_JPMorganChaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: