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Segment Information (Assets) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | --- | --- | ---
Segment reporting information |  |  |  |
Total investments | $ 6,886.3 us-gaap_Investments | $ 7,192.6 us-gaap_Investments |  |
Reinsurance recoverable on paid and unpaid losses | 507.5 us-gaap_ReinsuranceRecoverablesGross | 453.5 us-gaap_ReinsuranceRecoverablesGross |  |
Assets held for sale | 58.1 us-gaap_AssetsHeldForSaleAtCarryingValue | 1,880.1 us-gaap_AssetsHeldForSaleAtCarryingValue |  |
Assets | 10,456.9 us-gaap_Assets | 12,144.3 us-gaap_Assets |  |
Loss and loss adjustment expense reserves | 3,159.8 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | 3,079.3 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | 3,168.9 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | 5,702.3 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Liabilities held for sale | 0 wtm_LiabilitiesHeldForSale | 1,880.1 wtm_LiabilitiesHeldForSale |  |
Liabilities | 5,917.3 us-gaap_Liabilities | 7,746.9 us-gaap_Liabilities |  |
Non-controlling interests | 543.0 us-gaap_MinorityInterest | 491.9 us-gaap_MinorityInterest |  |
White Mountains’ common shareholders’ equity | 3,996.6 us-gaap_StockholdersEquity | 3,905.5 us-gaap_StockholdersEquity |  |
OneBeacon |  |  |  |
Segment reporting information |  |  |  |
Total investments | 2,527.0 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 2,364.9 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Reinsurance recoverable on paid and unpaid losses | 173.8 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 89.9 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Assets held for sale | 58.1 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,880.1 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Assets | 3,561.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 5,191.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Loss and loss adjustment expense reserves | 1,342.2 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1,054.3 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Liabilities held for sale |  | 1,880.1 wtm_LiabilitiesHeldForSale / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Liabilities | 2,510.5 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 4,083.9 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Non-controlling interests | 262.1 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 276.8 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
White Mountains’ common shareholders’ equity | 788.4 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 830.6 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Sirius Group |  |  |  |
Segment reporting information |  |  |  |
Total investments | 3,178.4 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 3,251.4 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Reinsurance recoverable on paid and unpaid losses | 333.6 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 363.6 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Assets held for sale | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Assets | 5,034.7 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 5,124.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Loss and loss adjustment expense reserves | 1,809.8 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 2,025.0 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Liabilities held for sale |  | 0 wtm_LiabilitiesHeldForSale / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Liabilities | 3,180.6 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 3,414.5 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Non-controlling interests | 250.0 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 250.0 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
White Mountains’ common shareholders’ equity | 1,604.1 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1,459.6 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
HG Global |  |  |  |
Segment reporting information |  |  |  |
Total investments | 121.0 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 107.9 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Reinsurance recoverable on paid and unpaid losses | 0 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 0 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Assets held for sale | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Assets | 704.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 675.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Loss and loss adjustment expense reserves | 0 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 0 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Liabilities held for sale |  | 0 wtm_LiabilitiesHeldForSale / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Liabilities | 120.9 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 68.9 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Non-controlling interests | 17.9 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 16.6 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
White Mountains’ common shareholders’ equity | 565.5 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 589.5 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
BAM |  |  |  |
Segment reporting information |  |  |  |
Total investments | 454.2 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | 468.6 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Reinsurance recoverable on paid and unpaid losses | 0 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | 0 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Assets held for sale | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Assets | (89.4)us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | (77.9)us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Loss and loss adjustment expense reserves | 0 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | 0 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Liabilities held for sale |  | 0 wtm_LiabilitiesHeldForSale / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Liabilities | 32.5 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | 19.7 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Non-controlling interests | (121.9)us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | (97.6)us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
White Mountains’ common shareholders’ equity | 0 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | 0 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Surplus notes | 503.0 us-gaap_SurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |  |
Accrued interest on surplus notes | 74.4 wtm_AccruedInterestOnSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | 58.6 wtm_AccruedInterestOnSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Other Operations |  |  |  |
Segment reporting information |  |  |  |
Total investments | 605.7 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | 999.8 us-gaap_Investments / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
Reinsurance recoverable on paid and unpaid losses | 0.1 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | 0 us-gaap_ReinsuranceRecoverablesGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
Assets held for sale | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | 0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
Assets | 1,246.2 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | 1,231.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
Loss and loss adjustment expense reserves | 7.8 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | 0 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
Liabilities held for sale |  | 0 wtm_LiabilitiesHeldForSale / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
Liabilities | 72.8 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | 159.9 us-gaap_Liabilities / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
Non-controlling interests | 134.9 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | 46.1 us-gaap_MinorityInterest / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |
White Mountains’ common shareholders’ equity | $ 1,038.6 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember | $ 1,025.8 us-gaap_StockholdersEquity / us-gaap_StatementBusinessSegmentsAxis = wtm_OtherOperationsMember |  |

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