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Variable Annuity Reinsurance (Details 2) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2012
Variable annuity benefit guarantee liabilities |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Fair value Liabilities | 548.5
Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders (as a percent) | 0.10%
Mortality (as a percent) | 0.10%
Foreign exchange volatilities (as a percent) | 12.10%
Index volatilities (as a percent) | 11.90%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders (as a percent) | 3.00%
Mortality (as a percent) | 3.10%
Foreign exchange volatilities (as a percent) | 28.70%
Index volatilities (as a percent) | 30.20%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders (as a percent) | 0.70%
Mortality (as a percent) | 1.30%
Foreign exchange volatilities (as a percent) | 16.90%
Index volatilities (as a percent) | 22.80%
Foreign exchange and equity index options |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Fair value Liabilities | 183.8
Foreign exchange and equity index options | Black-Scholes option pricing model | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Foreign exchange volatilities (as a percent) | 12.10%
Index volatilities (as a percent) | 11.90%
Expected equity dividends (as a percent) | 1.40%
Foreign exchange and equity index options | Black-Scholes option pricing model | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Foreign exchange volatilities (as a percent) | 28.70%
Index volatilities (as a percent) | 30.20%
Expected equity dividends (as a percent) | 6.20%
Foreign exchange and equity index options | Black-Scholes option pricing model | Weighted Average |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Foreign exchange volatilities (as a percent) | 16.90%
Index volatilities (as a percent) | 22.80%
Expected equity dividends (as a percent) | 2.70%

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- Definition This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References No definition available. + Details
Name: | us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Represents the expected equity dividends, used as an input to measure fair value. + References No definition available. + Details
Name: | wtm_FairValueInputsExpectedEquityDividends
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the foreign exchange volatilities, used as an input to measure fair value. + References No definition available. + Details
Name: | wtm_FairValueInputsForeignExchangeVolatilities
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the index volatilities, used as an input to measure fair value. + References No definition available. + Details
Name: | wtm_FairValueInputsIndexVolatilities
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | wtm_FairValueInputsLiabilitiesQuantitativeInformationLineItems
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the mortality, used as an input to measure fair value. + References No definition available. + Details
Name: | wtm_FairValueInputsMortality
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the surrenders, used as an input to measure fair value. + References No definition available. + Details
Name: | wtm_FairValueInputsSurrenders
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration