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Investment Securities (Details 10) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
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

X

- 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

X

- 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

X

- Details

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

X

- 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