XML 83 R72.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Municipal Bond Guarantee (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2017 USD ($) Contract | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) yr Contract | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($) Contract | Dec. 31, 2012 USD ($) | Jun. 30, 2017 USD ($) | Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Pre-tax income (loss) | $ 8.4 | $ (13.8) | $ 2.6 | $ (72.8) |  |  |  |
Net investment income (loss) - BAM Surplus Note interest | 0.0 | 0.0 | 0.0 | 0.0 |  |  |  |
HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Partners capital contribution |  |  |  |  |  | $ 594.5 |  |
Surplus notes |  |  |  |  |  |  |  | $ 503.0
Income (loss) from continuing operations before income taxes, domestic | $ 7.0 |  | $ 20.3 |  |  |  |  |
HG Global | Preferred Stock |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 96.90% |  | 96.90% |  | 96.90% |  | 96.90% |
HG Global | Common Stock |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 88.40% |  | 88.40% |  | 88.40% |  | 88.40% |
BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Surplus notes | $ 500.0 |  | $ 500.0 |  |  |  |  |
Year of U.S. Treasury rate | yr |  |  | 1 |  |  |  |  |
Municipal bond contract | Contract | 5,907 |  | 5,907 |  | 4,807 |  |  |
Remaining weighted average contract period outstanding (in years) |  |  | 10 years 11 months |  | 10 years 10 months |  |  |
Municipal bond outstanding principal | $ 39,207.5 |  | $ 39,207.5 |  | $ 33,057.3 |  |  |
Municipal bond outstanding interest | 19,681.0 |  | 19,681.0 |  | 16,396.6 |  |  |
Municipal bond debt service outstanding | 58,888.5 |  | 58,888.5 |  | 49,453.9 |  |  |
October 1, 2017 - December 31, 2017 | 2.5 |  | 2.5 |  |  |  |  |
2018 | 9.9 |  | 9.9 |  |  |  |  |
2019 | 9.5 |  | 9.5 |  |  |  |  |
2020 | 9.2 |  | 9.2 |  |  |  |  |
2021 | 8.7 |  | 8.7 |  |  |  |  |
2022 and thereafter | 78.7 |  | 78.7 |  |  |  |  |
Total gross unearned insurance premiums | 118.5 |  | 118.5 |  | $ 82.9 |  |  |
Build America Mutual |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Income (loss) from continuing operations before income taxes, domestic | (11.9) |  | (35.6) |  |  |  |  |
HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Pre-tax income (loss) | 7.0 | 5.2 | 20.3 | 18.4 |  |  |  |
Net investment income (loss) - BAM Surplus Note interest | 4.8 | 4.5 | $ 14.3 | 13.4 |  |  |  |
HG Re |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  |  | 15.00% |  |  |  |  |
BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Pre-tax income (loss) | (11.9) | (13.7) | $ (35.6) | (30.3) |  |  |  |
Net investment income (loss) - BAM Surplus Note interest | (4.8) | $ (4.5) | (14.3) | $ (13.4) |  |  |  |
BAM Management |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Partners capital contribution |  |  |  |  |  | $ 14.5 |  |
Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Interest income, other | $ 4.8 |  | $ 14.3 |  |  |  |  |
Derivative, fixed interest rate (as a percent) | 3.78% |  | 3.78% |  | 3.54% |  |  |
Basis points |  |  | 30000.00% |  |  |  |  |
Surplus Note | Build America Mutual |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Interest expense | $ 4.8 |  | $ 14.3 |  |  |  |  |
Combination One | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Number of years of change in interest rate | yr |  |  | 5 |  |  |  |  |
Combination Two | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Number of years of change in interest rate | yr |  |  | 3 |  |  |  |  |
US Treasury Rate | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Basis points |  |  | 3.00% |  |  |  |  |
Fixed interest rate | Surplus Note | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) | 8.00% |  | 8.00% |  |  |  |  |
First Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2018 | $ 2.5 |  | $ 2.5 |  |  |  |  |
Second Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2018 | 2.5 |  | 2.5 |  |  |  |  |
Third Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2018 | 2.5 |  | 2.5 |  |  |  |  |
Fourth Quarter, year two | BAM |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
2018 | $ 2.4 |  | $ 2.4 |  |  |  |  |
Series A BAM Surplus Note | HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  | $ 203.0 |
Series B BAM Surplus Note | HG Global |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  | $ 300.0 |

X
- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of unearned premium revenue under contracts issued by an insurance entity that provide protection to the holder of a financial obligation from a financial loss in the event of a default. + References No definition available. + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of premium revenue under financial guarantee insurance contracts expected to be recognized in the sixth through tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 4 -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanFiveAndWithin10YearsFromBalanceSheetDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of premium revenue under financial guarantee insurance contracts expected to be recognized in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanFourAndWithinFiveYearsFromBalanceSheetDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of premium revenue under financial guarantee insurance contracts expected to be recognized in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanOneAndWithinTwoYearsFromBalanceSheetDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of premium revenue under financial guarantee insurance contracts expected to be recognized in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanThreeAndWithinFourYearsFromBalanceSheetDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of premium revenue under financial guarantee insurance contracts expected to be recognized in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanTwoAndWithinThreeYearsFromBalanceSheetDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of premium revenue under financial guarantee insurance contracts expected to be recognized in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedOneYearFromBalanceSheetDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_GuaranteeObligationsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The portion of earnings or loss from continuing operations before income taxes that is attributable to domestic operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(1)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
Name: | us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(1)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
Name: | us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Remaining weighted average contract period of financial guarantee insurance contracts, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 9 -Subparagraph (a)(5)(ii) -URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 55 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=34726416&loc=SL5751133-161288 + Details
Name: | us-gaap_InsuredFinancialObligationsWithCreditDeteriorationRemainingWeightedAverageContractPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of the cost of borrowed funds accounted for as interest expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.9) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
Name: | us-gaap_InterestExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of interest income earned from interest bearing assets classified as other. + References No definition available. + Details
Name: | us-gaap_InterestIncomeOther
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Total contributions made by each class of partners (i.e., general, limited and preferred partners). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 4.F) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
Name: | us-gaap_PartnersCapitalAccountContributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of risk reinsured on insurance products per entity policy. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486086&loc=d3e24387-158525 + Details
Name: | us-gaap_SurplusNotes
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Municipal bond contracts issued and outstanding. + References No definition available. + Details
Name: | wtm_MunicipalBondContract
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Municipal bond outstanding interest + References No definition available. + Details
Name: | wtm_MunicipalBondOutstandingInterest
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Municipal bond outstanding principal + References No definition available. + Details
Name: | wtm_MunicipalBondOutstandingPrincipal
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Municipal bond debt service outstanding + References No definition available. + Details
Name: | wtm_Municipalbonddebtserviceoutstanding
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Net Investment Income, Surplus Notes + References No definition available. + Details
Name: | wtm_NetInvestmentIncomeSurplusNotes
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of years of change in interest rate + References No definition available. + Details
Name: | wtm_Numberofyearsofchangeininterestrate
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Year of U.S. Treasury Rate + References No definition available. + Details
Name: | wtm_YearofU.S.TreasuryRate
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=wtm_HgGlobalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_PreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_BamMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_BuildAmericaMutualMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=wtm_BamManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=wtm_SurplusNoteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentRedemptionPeriodAxis=wtm_CombinationOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentRedemptionPeriodAxis=wtm_CombinationTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=wtm_USTreasuryRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=wtm_FixedinterestrateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_FuturePremiumsAxis=wtm_FirstQuarteryeartwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_FuturePremiumsAxis=wtm_SecondQuarteryeartwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_FuturePremiumsAxis=wtm_ThirdQuarteryeartwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_FuturePremiumsAxis=wtm_FourthQuarteryeartwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=wtm_SeriesABAMSurplusNoteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=wtm_SeriesBBAMSurplusNoteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |