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Derivatives Derivatives (Interest Rate Cap) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 1 Months Ended
--- | --- | ---
Dec. 31, 2014 Rating | Dec. 31, 2013 | May 31, 2007
Interest Rate Cap |  |  |
Derivative [Line Items] |  |  |
Derivative Collateral Right to Reclaim | $ 4.3 wtm_DerivativeCollateralRightToReclaim / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |  |
Beginning of period | 11.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember | 0 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |
Purchases | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember | 9.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |
Change in fair value of derivatives, included in other revenue | 7.0 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember | (1.2)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |
Ending balance | 4.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember | 11.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |
S&P Credit Ratings | 21 wtm_SAndPCreditRatings / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |  |
Barclays [Member] | A | Interest Rate Cap |  |  |
Derivative [Line Items] |  |  |
Derivative Collateral Right to Reclaim | 3.0 wtm_DerivativeCollateralRightToReclaim / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |  |
Nordea Bank Finland [Member] | Standard & Poor's, AA- Rating [Member] | Interest Rate Cap |  |  |
Derivative [Line Items] |  |  |
Derivative Collateral Right to Reclaim | 1.3 wtm_DerivativeCollateralRightToReclaim / us-gaap_CounterpartyNameAxis = wtm_NordeaBankFinlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAAMinusRatingMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |  |
Sirius Internation Group Ltd | Interest Rate Cap |  |  |
Derivative [Line Items] |  |  |
basis points | 3.95%wtm_Basispoints / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember |  |
years of interest rate cap | 5 years | 5 years |
Derivative, Cap Interest Rate | 8.30%us-gaap_DerivativeCapInterestRate / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember | 8.30%us-gaap_DerivativeCapInterestRate / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember |
Derivative, Cost of Hedge | $ 9.9 us-gaap_DerivativeCostOfHedge / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember |  |
Sirius Internation Group Ltd | Preferred stocks |  |  |
Derivative [Line Items] |  |  |
Non-cumulative perpetual preference shares issued |  |  | 250,000,000 us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember
Sirius Internation Group Ltd | Preferred stocks | Interest Rate Cap |  |  |
Derivative [Line Items] |  |  |
Preferred stock dividend rate |  |  | 7.506%us-gaap_PreferredStockDividendRatePercentage / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember
Preference shares reference rate for dividend |  |  | 3
Non-cumulative perpetual preference shares issued |  |  | 250,000,000 us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember / dei_LegalEntityAxis = wtm_SiriusGroupLtdMember / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredStockMember

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- Definition Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract. + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition The premium (cost) of a hedge, expensed during the period. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References 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
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- 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
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- Details
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- 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
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | duration

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- Definition The percentage rate used to calculate dividend payments on preferred stock. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Definition The number of shares issued or sold by the subsidiary or equity method investee per stock transaction. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition basis points + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of collateral held against amounts due to counterparties for over the counter derivative instruments against net derivative instrument positions. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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
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- Definition Represents the reference rate used as a basis in determining the floating annual rate of dividend on the preference shares. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
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- 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

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- Definition Years of interest rate cap + References No definition available. + Details
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- Details
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- Details
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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