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Investment Securities (Details 10) (USD $) In Millions, unless otherwise specified | Mar. 31, 2012
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Agency RMBS | Discounted cash flows | AA+ |
Quantitative information for Level 3 Fair Value Measurements Assets |
Fair value Assets | $ 73.2
Prepayment rate (as a percent) | 15.00%
Discount Yield (as a percent) | 2.10%
Non-agency RMBS | Discounted cash flows | CCC |
Quantitative information for Level 3 Fair Value Measurements Assets |
Fair value Assets | 15.1
Prepayment rate (as a percent) | 10.00%
Discount Yield (as a percent) | 8.00%
Loss severity (as a percent) | 40.00%
Preferred Stock | Discounted cash flows | NR |
Quantitative information for Level 3 Fair Value Measurements Assets |
Fair value Assets | 69.8
Discount Yield (as a percent) | 8.80%
Corporate Bonds | Broker pricing | BBB+ |
Quantitative information for Level 3 Fair Value Measurements Assets |
Fair value Assets | 21.0
Broker quote | $ 21.0

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- Definition This element represents the aggregate of the assets 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_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents fair value of broker quote. + References No definition available. + Details
Name: | wtm_FairValueBrokerQuote
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Definition Represents the constant prepayment rate, used as an input to measure fair value. + References No definition available. + Details
Name: | wtm_FairValueInputsConstantPrepaymentRate
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References No definition available. + Details
Name: | wtm_FairValueInputsDiscountRate
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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