XML 52 R44.htm IDEA: XBRL DOCUMENT
v3.3.0.814

Loss and Loss Adjustment Expense Reserves Details (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
Net (favorable) and unfavorable loss reserve development | $ (0.4) | $ 7.1 | $ (2.6) | $ 16.3
Star & Shield Insurance Exchange |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
Net (favorable) and unfavorable loss reserve development | $ (0.4) | (0.2) | (0.8) | 2.0
OneBeacon |  |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |  |
Net (favorable) and unfavorable loss reserve development |  | $ 7.3 | $ (1.8) | $ 14.3

X
- Definition Amount of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 + Details
Name: | us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | wtm_LossAndLossAdjustmentExpenseReservesLineItems
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_StarShieldInsuranceExchangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_OnebeaconMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |