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Derivatives Derivative (Tranzact Interest Rate Swap) (Details) (Details) (Tranzact [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Oct. 10, 2014 | Dec. 31, 2013 | Mar. 31, 2014
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Tranzact Bank Facility [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Line of Credit Facility, Fair Value of Amount Outstanding | $ 67.4 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember | $ 70.0 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember | $ 0 us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |
Line of Credit Facility, Amount Outstanding | 68.7 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  | 0 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate | 4.15575%us-gaap_LongTermDebtPercentageBearingVariableInterestRate / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  |
Minimum | Tranzact Bank Facility [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
LIBOR rate | 2.50%wtm_LIBORrate / us-gaap_DebtInstrumentAxis = wtm_TranzactBankFacilityMember / dei_LegalEntityAxis = wtm_TranzactMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |
Interest Rate Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Fixed Interest Rate | 1.34%us-gaap_DerivativeFixedInterestRate / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / dei_LegalEntityAxis = wtm_TranzactMember |  |  |
Derivative, Variable Interest Rate | 0.1558%us-gaap_DerivativeVariableInterestRate / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / dei_LegalEntityAxis = wtm_TranzactMember | 0.1518%us-gaap_DerivativeVariableInterestRate / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / dei_LegalEntityAxis = wtm_TranzactMember |  |
Derivative, Fair Value, Net |  |  |  | $ 0.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / dei_LegalEntityAxis = wtm_TranzactMember

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

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- Definition Fixed interest rate related to the interest rate derivative. + References No definition available. + Details
Name: | us-gaap_DerivativeFixedInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Variable interest rate in effect as of the balance sheet date related to the interest rate derivative. + References No definition available. + Details
Name: | us-gaap_DerivativeVariableInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Amount borrowed under the credit facility as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
Name: | us-gaap_LineOfCreditFacilityAmountOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Fair value of the amount outstanding under the credit facility. + 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 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13476-108611 + Details
Name: | us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtPercentageBearingVariableInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition LIBOR rate + References No definition available. + Details
Name: | wtm_LIBORrate
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |