Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Mar. 31, 2015 Rating	Dec. 31, 2014
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 47.0	$ 56.4
Collertal provided to counterparty in cash	1.6	3.5
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	48.6	59.9
Excess collateral provided to counterparty in cash	27.4	17.7
Derivative, Collateral, Right to Reclaim Securities	4.5	9.5
Counter-party collateral held by WMLife Re - Cash	7.4	9.9
Net amount of exposure to counter-party	73.1	77.2
S and P Credit Ratings	23wtm_SAndPCreditRatings
Standard & Poor's, A Rating \| Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	4.9	5.6
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	4.9	5.6
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	4.9	5.6
Standard & Poor's, A Rating \| Barclays
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	0	0.1
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	0.1
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	0	0.1
Standard & Poor's, A Rating \| Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	18.6	22.2
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	18.6	22.2
Excess collateral provided to counterparty in cash	0.7	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	1.1
Net amount of exposure to counter-party	19.3	21.1
Standard & Poor's, A Rating \| Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(1.2)	3.7
Collertal provided to counterparty in cash	1.2	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	3.7
Excess collateral provided to counterparty in cash	21.1	16.0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	21.1	19.7
Standard & Poor's, A plus \| JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	21.3	24.3
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	21.3	24.3
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	7.4	8.8
Net amount of exposure to counter-party	13.9	15.5
Standard & Poor's, BBB+ Rating [Member] \| Nomura [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(0.4)	(3.5)
Collertal provided to counterparty in cash	0.4	3.5
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	0
Excess collateral provided to counterparty in cash	5.6	1.7
Derivative, Collateral, Right to Reclaim Securities	4.5	9.5
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	10.1	11.2
Standard & Poor's, A- Rating \| Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	3.8	4.0
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	3.8	4.0
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	$ 3.8	$ 4.0

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:	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: