XML 113 R65.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 9 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2014
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | $ (6.0)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears | $ (48.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears | $ (41.9)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears |  |
Increase (decrease) in reserves | 1,169.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease | 1,040.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease | 1,193.9 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease |  |
Current Accident Year | 1,175.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear | 1,088.9 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear | 1,235.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear |  |
Accretion of fair value adjustment to net loss and LAE reserves | 0.7 wtm_AccretedFairValueAdjustmentToLossAndLossAdjustmentExpenseReserves | 1.7 wtm_AccretedFairValueAdjustmentToLossAndLossAdjustmentExpenseReserves | 10.6 wtm_AccretedFairValueAdjustmentToLossAndLossAdjustmentExpenseReserves |  |
Un-accreted fair value, adjustment to loss and loss adjustment expense reserves, before tax | 4.0 wtm_UnaccretedFairValueAdjustmentToLossAndLossAdjustmentExpenseReservesBeforeTax |  |  |  |
Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | (98.0)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (23.9)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (34.5)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Liability for asbestos and environmental claims, gross | 238.5 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 227.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 228.6 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 207.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Liability for asbestos claims, net, period increase (decrease) | 8.0 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 11.8 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 33.0 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 46.4 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Liability for asbestos and environmental claims | 210.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 193.9 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 189.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 162.7 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Liability for Asbestos Claims, Net, Period Increase (Decrease) Based on Quarterly Reporting |  |  | 14.0 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecreaseBasedonQuarterlyReporting / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Liabilty for environmental claims, net, period increase (decrease) | 1.6 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0.8 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0.5 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Asbestos and Environmental Loss Three Year Survival Ratio Period | 9 years | 8 years |  |  |
OneBeacon |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | 75.5 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  | (7.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  | 14.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Increase (decrease) in reserves | 109.2 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember | 89.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |
Current Accident Year | 33.7 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  | 33.7 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember
Liability for asbestos and environmental claims, gross | 1.0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | 2.2 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_StatementBusinessSegmentsAxis = wtm_StarShieldInsuranceExchangeMember |  |  |  |
Asbestos | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Liability for asbestos and environmental claims, gross | 215.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 207.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 208.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 185.1 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Liability for asbestos and environmental claims | 192.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 178.7 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 174.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 146.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Environmental | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Liability for asbestos and environmental claims, gross | 22.7 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 20.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 20.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 22.1 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Liability for asbestos and environmental claims | 17.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 15.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 15.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 16.5 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
WM Solutions | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Liability for asbestos claims, net, period increase (decrease) | 22.9 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease / dei_LegalEntityAxis = wtm_WmSolutionsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 13.0 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease / dei_LegalEntityAxis = wtm_WmSolutionsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 11.0 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease / dei_LegalEntityAxis = wtm_WmSolutionsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Liabilty for environmental claims, net, period increase (decrease) | 2.1 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease / dei_LegalEntityAxis = wtm_WmSolutionsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1.0 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease / dei_LegalEntityAxis = wtm_WmSolutionsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0.7 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease / dei_LegalEntityAxis = wtm_WmSolutionsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Property [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | (54.0)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_PropertyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (16.6)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_PropertyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (28.4)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_PropertyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Professional Insurance | OneBeacon |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | 46.4 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_ProfessionalInsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
Increase (decrease) in reserves | 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 |  |  |
Current Accident Year | 22.9 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_ProfessionalInsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
Property, Liability and Casualty Insurance Product Line | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | (13.0)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |  |  |
asbestos and environmental [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | 10.0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AsbestosandenvironmentalMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |  |  |
Property Loss Held Above Actuarial Central Estimate [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | (24.0)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_PropertyLossHeldAboveActuarialCentralEstimateMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |  |  |
Aviation Space [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | 13.0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AviationSpaceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (10.1)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AviationSpaceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (5.3)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AviationSpaceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Accident and Health Insurance Product Line [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | (13.0)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_AccidentAndHealthInsuranceSegmentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (9.2)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_AccidentAndHealthInsuranceSegmentMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |  |
Crop | OneBeacon |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | 0 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
Increase (decrease) in reserves | 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 |  |  |
Current Accident Year | 3.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CropinsuranceMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
Specialty Products | OneBeacon |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year | 51.7 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
Increase (decrease) in reserves | 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 |  |  |
Current Accident Year | 28.4 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |
Asbestos [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year |  | 11.8 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AsbestosMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 46.4 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AsbestosMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Casualty Runoff [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year |  |  | (31.5)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_CasualtyRunoffMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Marine Energy [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year |  |  | (11.6)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_MarineEnergyMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Trade Credit [Member] | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year |  |  | (6.8)us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_TradeCreditMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Accident and Health | Sirius Group |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Prior Accident Year |  |  | 4.3 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_AccidentAndHealthMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |  |
Management's estimate [Member] | Specialty Products | OneBeacon |  |  |  |  |
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |
Current Accident Year | $ 29.9 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear / us-gaap_ChangeInAccountingEstimateByTypeAxis = wtm_ManagementsestimateMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = wtm_SpecialtyProductsMember / us-gaap_StatementBusinessSegmentsAxis = wtm_OnebeaconMember |  |  |  |

X
- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of provision in the period for claims incurred in the reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of provision in the period for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition The amount of the other adjustment made to the claims reserves during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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