XML 85 R56.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Basis of Presentation |  |
Assets held for sale | $ 58.1 us-gaap_AssetsHeldForSaleAtCarryingValue | $ 1,880.1 us-gaap_AssetsHeldForSaleAtCarryingValue
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured |
OneBeacon Ltd. |  |
Basis of Presentation |  |
Ownership interest (as a percent) | 75.30%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember | 75.20%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember
HG Global |  |
Basis of Presentation |  |
Surplus notes | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / dei_LegalEntityAxis = wtm_HgGlobalMember |
Star & Shield Insurance Exchange [Member] |  |
Basis of Presentation |  |
Surplus notes | 17.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |
Redeemable preferred stocks | HG Global |  |
Basis of Presentation |  |
Ownership interest (as a percent) | 96.90%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 97.30%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember
Common Stock | HG Global |  |
Basis of Presentation |  |
Ownership interest (as a percent) | 88.40%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 88.70%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Wholly Owned Properties [Member] | OneBeacon Ltd. |  |
Basis of Presentation |  |
Assets held for sale | $ 58.1 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_FairValueByAssetClassAxis = us-gaap_WhollyOwnedPropertiesMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |

X
- Definition Represents the sum of the carrying value of all the Assets Held-for-Sale at the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 + Details
Name: | us-gaap_AssetsHeldForSaleAtCarryingValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486086&loc=d3e24387-158525 + Details
Name: | us-gaap_SurplusNotes
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Definition Percentage of par value of policy reinsured. + References No definition available. + Details
Name: | wtm_PercentageOfParValueOfPolicyReinsured
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | dei_LegalEntityAxis=wtm_HgGlobalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_OneBeaconInsuranceGroupLtdMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_StarShieldInsuranceExchangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByAssetClassAxis=us-gaap_WhollyOwnedPropertiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_PreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |