Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	$ 0	$ 59.4
Changes in derivative instruments
Beginning of period	69.2	98.3	290.4
Purchases			6.1
Realized and unrealized gains (losses)	72.4	402.0	339.0
Transfers in (out)	0wtm_DerivativeTransfersInOutNet	0wtm_DerivativeTransfersInOutNet	0wtm_DerivativeTransfersInOutNet
Sales/settlements	59.6	313.5	140.8
Ending balance	56.4	69.2	98.3
Collateral Cash and Investments	33.2wtm_CollateralCashAndInvestments	81.3wtm_CollateralCashAndInvestments	393.6wtm_CollateralCashAndInvestments
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	0.6wtm_GainLossOnLiabilityForDeathBenefit	10.2wtm_GainLossOnLiabilityForDeathBenefit	14.2wtm_GainLossOnLiabilityForDeathBenefit
Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases		0
Changes in variable annuity (liabilities)
Purchases			0
Level 2
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0	0
Changes in derivative instruments
Beginning of period	4.7	(20.5)	39.2
Purchases			0
Realized and unrealized gains (losses)	71.0	196.1	186.9
Transfers in (out)	0	0	0
Sales/settlements	100.1	221.3	127.2
Ending balance	33.8	4.7	(20.5)
Level 1
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0	0
Changes in derivative instruments
Beginning of period	1.1	(21.7)	4.1
Purchases			0
Realized and unrealized gains (losses)	(37.2)	69.4	68.1
Transfers in (out)	0	0	0
Sales/settlements	(34.6)	92.2	42.3
Ending balance	3.7	1.1	(21.7)
Variable Annuity [Member] \| Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0
Changes in variable annuity (liabilities)
Beginning balance	(52.8)	(441.5)	(768.5)
Realized and unrealized gains (losses)	(53.5)	(388.7)	(327.0)
Transfers in (out)	0	0	0
Sales/settlements	0	0	0
Ending balance	0.7	(52.8)	(441.5)
Derivative [Member] \| Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0	59.4
Changes in derivative instruments
Beginning of period	63.4	140.5	247.1
Purchases			6.1
Realized and unrealized gains (losses)	38.6	136.5	84.0
Transfers in (out)	0	0	0
Sales/settlements	(5.9)	0	(28.7)
Ending balance	$ 18.9	$ 63.4	$ 140.5

Fair value of the assets less the liabilities of a derivative or group of derivatives.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13495-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

+ Details

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

- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (a),(c),(d),(e)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959

+ Details

Name:	us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

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

- Definition

Represents cash and short-term investments held and posted as collateral to reinsurance counterparties.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

Derivative Transfers In (Out), Net

+ References

No definition available.

+ Details

Name:	wtm_DerivativeTransfersInOutNet
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Purchases that have taken place during the period in relation to derivative assets measured at fair value on a recurring basis.

+ References

No definition available.

+ Details

Name:	wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Sales and settlements that have taken place during the period in relation to derivative assets measured at fair value on a recurring basis.

+ References

No definition available.

+ Details

Name:	wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers

+ References

No definition available.

+ Details

Name:	wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfers
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sales or settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

No definition available.

+ Details

Name:	wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySalesOrSettlements
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Net increase or decrease in the fair value of the variable annuity death benefit liabilities included in earnings.

+ References

No definition available.

+ Details

Name:	wtm_GainLossOnLiabilityForDeathBenefit
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

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

- Details

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