XML 132 R90.htm IDEA: XBRL DOCUMENT
v2.4.0.6

Investment Securities (Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Number of Investments with Significant Unobseravble Inputs | 1 |  |
Moody's, A2 Rating | Agency CMBS | Broker Pricing |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Fair value Assets | $ 22.1 | [1] |
Standard Poors NR Rating | Redeemable preferred stocks | Discounted cash flows |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Fair value Assets | 70.8 | [1] |
Standard Poors NR Rating | Redeemable preferred stocks | Discount Yield |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Discount Yield (as a percent) | 7.60% |  |
Standard Poors NR Rating | Redeemable preferred stocks | Multiple of GAAP book value |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Book Value Multiple to Calculate Fair Value | 100.00% |  |
Standard Poors NR Rating | Private equity securities | Multiple of GAAP book value |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Fair value Assets | 36.9 |  |
Fixed maturity investments | Level 3 Inputs |  |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |  |
Transfers in | $ 22.1 |  | $ 18.2
[1] | As of December 31, 2012 each asset type consists of one security.

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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueInputsDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Book Value Multiple to Calculate Fair Value + References No definition available. + Details
Name: | wtm_BookValueMultipleToCalculateFairValue
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
Name: | wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersIn
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of Investments with Significant Unobseravble Inputs + References No definition available. + Details
Name: | wtm_NumberOfInvestmentsWithSignificantUnobseravbleInputs
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant