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v2.4.1.9

Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2014 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Premiums earned |  |  |  |
Current Accident Year |  | $ 1,175.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear | $ 1,088.9 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear | $ 1,235.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear
Prior Accident Year |  | (6.0)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears | (48.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears | (41.9)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears
Total incurred losses and LAE |  | 1,169.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease | 1,040.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease | 1,193.9 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease
OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year | 33.7 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 33.7 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year | 14.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 75.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  | (7.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Total incurred losses and LAE |  | 109.2 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 89.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Professional Insurance | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 22.9 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_ProfessionalInsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 46.4 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_ProfessionalInsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 69.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_ProfessionalInsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 59.1 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_ProfessionalInsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Specialty Property | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | (1.1)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyPropertyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 5.7 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyPropertyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 4.6 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyPropertyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1.1 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyPropertyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Crop | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 3.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 3.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Other | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 2.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | (0.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 2.4 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 1.6 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Specialty Products | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 28.4 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 51.7 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 80.1 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 61.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Entertainment | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 1.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_EntertainmentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 11.6 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_EntertainmentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 13.1 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_EntertainmentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 13.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_EntertainmentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Accident | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AccidentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 3.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AccidentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 3.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AccidentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 6.0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AccidentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Government Risks | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 1.2 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_GovernmentRisksMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 7.1 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_GovernmentRisksMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 8.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_GovernmentRisksMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 8.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_GovernmentRisksMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Other | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 2.6 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 1.6 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | 4.2 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_OtherSpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |
Specialty Industries | OneBeacon |  |  |  |
Premiums earned |  |  |  |
Current Accident Year |  | 5.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Prior Accident Year |  | 23.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |
Total incurred losses and LAE |  | $ 29.1 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | $ 28.0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyIndustriesMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |

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