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Data Type:	xbrli:stringItemType
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Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

No definition available.

Name:	us-gaap_TradingSecuritiesCost
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Data Type:	xbrli:monetaryItemType
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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities.

No definition available.

Name:	us-gaap_TradingSecuritiesUnrealizedHoldingGain
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Data Type:	xbrli:monetaryItemType
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Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities.

No definition available.

Name:	us-gaap_TradingSecuritiesUnrealizedHoldingLoss
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Data Type:	xbrli:monetaryItemType
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Represents the fair value of trading debt securities including assets classified as held for sale.

No definition available.

Name:	wtm_TradingSecuritiesDebtIncludingAssetsHeldForSale
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Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
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Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
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