Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	$ 56.4	$ 69.2
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	77.2	135.7
Derivative, Collateral, Right to Reclaim Cash	3.5	0
Derivative, Collateral, Right to Reclaim Securities	0	0.40
Derivative Asset, Fair Value, Amount Offset Against Collateral	59.9	69.6
Excess collateral provided to counterparty in cash	17.7	44.1
Excess collateral providedto counterparty in financial instruments	9.5	22.8
Derivative, Collateral, Obligation to Return Cash	9.9	0.8
Forward contracts
Credit risk on derivative instruments
S&P Credit Ratings	21
A \| Citigroup OTC [Member] \| White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	22.2	19.4
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	21.1	21.7
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	22.2	19.4
Excess collateral provided to counterparty in cash	0	2.3
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	1.1	0
A \| Barclays [Member] \| White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	0.1	1.4
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	0.1	1.4
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.1	1.4
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
A \| Bank of America [Member] \| White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	5.6	27.2
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	5.6	27.2
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	5.6	27.2
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
A \| Citigroup Exchange Traded [Member] \| White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	3.7	1.2
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	19.7	21.0
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	3.7	1.2
Excess collateral provided to counterparty in cash	16.0	19.8
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
BBB plus \| Nomura [Member] \| White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	(3.5)	(0.4)
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	11.2	22.0
Derivative, Collateral, Right to Reclaim Cash	3.5	0
Derivative, Collateral, Right to Reclaim Securities	0	0.40
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Excess collateral provided to counterparty in cash	1.7	0
Excess collateral providedto counterparty in financial instruments	9.5	22.8
Derivative, Collateral, Obligation to Return Cash	0	0.8
Standard & Poor's, A+ Rating [Member] \| J P Morgan Chase [Member] \| White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	24.3	9.1
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	15.5	31.1
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	24.3	9.1
Excess collateral provided to counterparty in cash	0	22.0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	8.8	0
Standard & Poor's, A- Rating [Member] \| Royal Bank of Scotland [Member] \| White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Net amounts recognized in Other Assets	4.0	11.3
Credit risk on derivative instruments
Net exposure on fair value of OTC instruments	4.0	11.3
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.0	11.3
Excess collateral provided to counterparty in cash	0	0
Excess collateral providedto counterparty in financial instruments	0	0
Derivative, Collateral, Obligation to Return Cash	$ 0	$ 0

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 210

-SubTopic 20

-Section 55

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=28370219&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=20225523&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=7476318&loc=d3e41678-113959

+ 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

+ 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

- 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

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=7476318&loc=d3e41678-113959

+ Details

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

- Definition

Fair value 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 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312

Reference 2: 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=28370219&loc=SL20226016-175313

Reference 3: 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=7476318&loc=d3e41678-113959

+ 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

- 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_StandardPoorsAMinusRatingMember
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_CreditRatingStandardPoorsAxis=us-gaap_StandardPoorsARatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: