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v2.4.1.9

Non-controlling Interest (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Noncontrolling Interest [Line Items] |  |
Total non-controlling interests | $ 528.5 us-gaap_MinorityInterest | $ 542.7 us-gaap_MinorityInterest
Total Non-Controlling Excluding Reciprocals [Member] |  |
Noncontrolling Interest [Line Items] |  |
Total non-controlling interests | 160.2 us-gaap_MinorityInterest / us-gaap_StatementEquityComponentsAxis = wtm_TotalNonControllingExcludingReciprocalsMember | 168.6 us-gaap_MinorityInterest / us-gaap_StatementEquityComponentsAxis = wtm_TotalNonControllingExcludingReciprocalsMember
OneBeacon Ltd. |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 24.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember | 24.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember
Total non-controlling interests | 258.8 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |
OneBeacon |  |
Noncontrolling Interest [Line Items] |  |
Total non-controlling interests | 258.8 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_OnebeaconMember | 258.4 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_OnebeaconMember
SIG Preference Shares |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember
Total non-controlling interests | 250.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember | 250.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_SIGPreferenceSharesMember
HG Global |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 3.10%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember | 3.10%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember
Total non-controlling interests | 17.7 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember | 17.9 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_HgGlobalMember
QuoteLab |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 40.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_QuoteLabMember | 40.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_QuoteLabMember
Total non-controlling interests | 22.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_QuoteLabMember | 22.6 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_QuoteLabMember
Tranzact [Member] |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 36.80%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_TranzactMember | 36.80%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_TranzactMember
Total non-controlling interests | 81.8 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_TranzactMember | 88.2 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_TranzactMember
WOBI |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 36.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WOBIMember | 36.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WOBIMember
Total non-controlling interests | 4.7 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WOBIMember | 5.4 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WOBIMember
Prospector Offshore Fund Ltd [Member] |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 32.40%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember | 23.40%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember
Total non-controlling interests | 31.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember | 31.1 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_ProspectorOffshoreFundLtdMember
A W G Dewar Inc [Member] |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 20.30%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WGDewarIncMember | 18.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_WGDewarIncMember
Total non-controlling interests | 3.0 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WGDewarIncMember | 3.4 us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_WGDewarIncMember
Build America Mutual |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember
Total non-controlling interests | (127.0)us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember | (121.9)us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember
Star & Shield Insurance Exchange |  |
Noncontrolling Interest [Line Items] |  |
Noncontrolling equity percentage | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember
Total non-controlling interests | $ (13.5)us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember | $ (12.4)us-gaap_MinorityInterest / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember

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- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which is directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent (that is, noncontrolling interest, previously referred to as minority interest). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.31) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 27 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 20 -Article 7 + Details
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- Definition The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity. + References No definition available. + Details
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