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v2.4.1.9

Reserves for Unpaid Losses and Loss Adjustment Expenses (LAE Reserve Activity) (Details) (Sirius Group, USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
Liability for asbestos claims, net, period increase (decrease) | $ 8.0 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease | $ 11.8 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease | $ 33.0 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease | $ 46.4 wtm_LiabilityforAsbestosClaimsNetPeriodIncreaseDecrease
Liabilty for environmental claims, net, period increase (decrease) | 1.6 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease | 0.8 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease | 0.5 wtm_LiabiltyForEnvironmentalClaimsNetPeriodIncreaseDecrease |
Gross |  |  |  |
Beginning balance | 227.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | 228.6 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | 207.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross |
Losses and LAE acquired | 25.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross | 14.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross | 11.7 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross |
Incurred losses and LAE | 9.0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss | 12.9 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss | 46.7 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss |
Paid losses and LAE | (23.3)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid | (27.7)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid | (37.0)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid |
Ending balance | 238.5 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | 227.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | 228.6 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | 207.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Net |  |  |  |
Beginning balance | 193.9 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 189.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 162.7 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet |
Losses and LAE acquired Central National | 25.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet | 14.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet | 11.7 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet |
Incurred losses and LAE | 9.6 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss | 12.6 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss | 45.9 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss |
Liability for unpaid claims and claims adjustment expense | (18.3)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid | (22.1)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid | (30.9)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid |
Ending balance | 210.2 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 193.9 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 189.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 162.7 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
Asbestos |  |  |  |
Gross |  |  |  |
Beginning balance | 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 |
Losses and LAE acquired | 22.9 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 13.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 11.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Incurred losses and LAE | 7.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 12.1 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 46.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Paid losses and LAE | (21.9)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (25.9)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (34.7)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Ending balance | 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 |
Net |  |  |  |
Beginning balance | 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 |
Losses and LAE acquired Central National | 22.9 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 13.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 11.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Incurred losses and LAE | 8.0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 11.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 46.4 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Liability for unpaid claims and claims adjustment expense | (16.8)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (20.3)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (29.4)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Ending balance | 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 |
Environmental |  |  |  |
Gross |  |  |  |
Beginning balance | 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 |
Losses and LAE acquired | 2.1 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0.7 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredGross / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Incurred losses and LAE | 1.6 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (0.1)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Paid losses and LAE | (1.4)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (1.8)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (2.3)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Ending balance | 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 |
Net |  |  |  |
Beginning balance | 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 |
Losses and LAE acquired Central National | 2.1 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 1.0 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0.7 wtm_LiabilityForAsbestosAndEnvironmentalClaimsAcquiredNet / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Incurred losses and LAE | 1.6 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | 0.8 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (0.5)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Liability for unpaid claims and claims adjustment expense | (1.5)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (1.8)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember | (1.5)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / us-gaap_CausesOfIncreaseDecreaseInLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_EnvironmentalIssueMember / us-gaap_StatementBusinessSegmentsAxis = wtm_SiriusGroupMember |
Ending balance | 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 |
WM Solutions |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
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 |

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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X
- Definition Amount of asbestos claims paid in the current period, 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 (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14784-158437 + Details
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- Definition Increase in asbestos and environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, 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 (b) -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 asbestos claims paid in the current period, net of 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 (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Increase in asbestos and environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, 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 (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Increase in asbestos and environmental reserve due to acquisition of Central National, before estimated recoveries from reinsurers. + References No definition available. + Details
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- Definition Increase in asbestos and environmental reserve due to acquisition of Central National, after estimated recoveries from reinsurers. + References No definition available. + Details
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- Definition Liability for Asbestos Claims, Net, Period Increase (Decrease) + References No definition available. + Details
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- Definition Liabilty for Environmental Claims, Net, Period Increase (Decrease) + References No definition available. + Details
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- Details
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- Details
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