XML 78 R67.htm IDEA: XBRL DOCUMENT
v3.20.2

Municipal Bond Guarantee - Narrative (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jan. 01, 2014 | Jan. 31, 2020 | Nov. 30, 2018 | Apr. 30, 2018 | Mar. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2012 | Dec. 31, 2024 | Jun. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2017 | Dec. 31, 2013 | Jul. 31, 2012
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent change in discounted future cash flows |  |  |  |  |  | 10.00% |  |  |  |  |  |  |  |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collateral held in supplement trust |  |  |  |  |  | $ 813.8 | $ 786.7 |  |  |  |  |  |  |  |
Interest receivable |  |  |  |  |  | $ 159.8 | 162.7 |  |  |  |  |  |  |  |
BAM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  | 457.6 |  |  |  | $ 409.7 |  |  |  |
Interest receivable |  |  |  |  |  |  | $ 7.5 |  |  |  | 4.7 |  |  |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital invested |  |  |  |  |  |  |  |  | $ 594.5 |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 503.0
Cash and Fixed Income Securities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.0
Percent over a target trust balance |  |  |  |  |  | 102.00% |  |  |  |  |  |  |  |  |
Supplement Trust Target |  |  |  |  |  | $ 603.0 |  |  |  |  |  |  |  |  |
HG Global | Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  | 96.90% | 96.60% |  |  |  |  |  |  |  |
HG Global | Common equity securities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  | 88.40% | 89.60% |  |  |  |  |  |  |  |
HG Global | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 503.0
Percentage of par value of policy reinsured |  |  |  |  |  | 15.00% |  |  |  |  |  |  |  |  |
HG Global | Series B BAM Surplus Note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 300.0
HG Global | Series A BAM Surplus Note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  | $ 203.0 |  | $ 203.0
HG Global | BAM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of Agreement |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |  |
HG Global | BAM | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate losses on ceded business |  | $ 75.0 |  |  |  |  |  |  |  |  |  |  |  |  |
HG Global | Non-controlling interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital invested |  |  |  |  |  |  |  |  | $ 14.5 |  |  |  |  |  |
BAM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of years of change in interest rate |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |
Municipal bond outstanding principal |  |  |  |  |  | $ 70,954.1 | $ 62,250.5 |  |  |  |  | $ 1,100.0 |  |  |
Additional municipal bond guarantee |  |  |  |  |  |  |  |  |  |  | $ 36.9 |  |  |  |
BAM | Supplemental Trust |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of Unsecured Debt |  |  |  |  | $ 47.9 |  |  |  |  |  |  |  |  |  |
Accrued Interest Payment on Surplus Notes |  |  |  |  | 0.9 |  |  |  |  |  |  |  |  |  |
BAM | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of risk premium ceded |  |  |  |  |  | 60.00% |  |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.00% |
Repayments of Unsecured Debt |  |  |  |  | 65.0 |  |  |  |  |  |  |  |  |  |
Accrued Interest Payment on Surplus Notes |  |  |  |  | $ 16.2 |  |  |  |  |  |  |  |  |  |
BAM | US Treasury (UST) Interest Rate | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 3.00% |  |  |  |  | 4.60% | 5.70% |  |  |  |  |  |  |  |
BAM | Forecast | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  |  |  |  |  |  |  |  |  | 8.00% |  |  |  |  |
BAM | Fidus Re Ltd. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt |  |  |  | $ 100.0 |  |  |  |  |  |  |  |  |  |  |
Collateralized financial guarantee, term |  |  |  | 12 years |  |  |  |  |  |  |  |  |  |  |
Collateral financial guarantee, callable term |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |
Aggregate losses on ceded business |  |  |  | $ 165.0 |  |  |  |  |  |  |  |  |  |  |
Premium receivable, Premium expected to be collected |  |  |  | 100.0 |  |  |  |  |  |  |  |  |  |  |
Municipal bond outstanding principal |  |  | $ 2,200.0 |  |  |  |  |  |  |  |  |  |  |  |
Percentage of quota share reinsurance agreement |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |
BAM | Fidus Re Ltd. | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate losses on ceded business |  |  |  | $ 276.1 |  |  |  |  |  |  |  |  |  |  |
Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Losses above retention, percentage of liability |  |  |  | 90.00% |  |  |  |  |  |  |  |  |  |  |

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://fasb.org/us-gaap/role/ref/legacyRef -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=120391452&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&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 premium receivables under financial guarantee insurance contracts. + References No definition available. + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivablePremiumExpectedToBeCollected
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.8) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_InterestReceivable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of funds committed to investment company from investors. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 205 -Section 50 -Paragraph 25 -URI http://asc.fasb.org/extlink&oid=25866437&loc=d3e10491-115837 + Details
Name: | us-gaap_InvestmentCompanyCommittedCapital
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of expected guaranteed benefits in excess of the projected account balances for minimum guaranteed benefit contracts, as of the balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 55 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=121639652&loc=d3e7104-158389 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 80 -Section 50 -Paragraph 1 -Subparagraph (d)(3) -URI http://asc.fasb.org/extlink&oid=121641442&loc=d3e19393-158473 + Details
Name: | us-gaap_LiabilitiesForGuaranteesOnLongDurationContractsGuaranteedBenefitLiabilityGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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 The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585 + Details
Name: | us-gaap_RepaymentsOfUnsecuredDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_SecuredDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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://fasb.org/us-gaap/role/ref/legacyRef -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 Accrued interest payment on surplus notes issued by VIE. The interest is eliminated in consolidation with the primary beneficiary. + References No definition available. + Details
Name: | wtm_AccruedInterestPaymentonSurplusNotes
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Additional Municipal Bond Guarantee + References No definition available. + Details
Name: | wtm_AdditionalMunicipalBondGuarantee
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Cash and Fixed Income Securities + References No definition available. + Details
Name: | wtm_CashandFixedIncomeSecurities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Collateral Financial Guarantee, Callable Term + References No definition available. + Details
Name: | wtm_CollateralFinancialGuaranteeCallableTerm
Namespace Prefix: | wtm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Collateral Held in Supplement Trust + References No definition available. + Details
Name: | wtm_CollateralHeldinSupplementTrust
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Collateralized Financial Guarantee, Term + References No definition available. + Details
Name: | wtm_CollateralizedFinancialGuaranteeTerm
Namespace Prefix: | wtm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Losses Above Retention, Percentage of Liability + References No definition available. + Details
Name: | wtm_LossesAboveRetentionPercentageofLiability
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Represents the percentage ceded under quota share reinsurance agreement. + References No definition available. + Details
Name: | wtm_PercentageOfQuotaShareReinsuranceAgreement
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of Risk Premium Ceded + References No definition available. + Details
Name: | wtm_PercentageofRiskPremiumCeded
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percent change in discounted future cash flows + References No definition available. + Details
Name: | wtm_Percentchangeindiscountedfuturecashflows
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percent over a target trust balance + References No definition available. + Details
Name: | wtm_Percentoveratargettrustbalance
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Supplement Trust Target + References No definition available. + Details
Name: | wtm_SupplementTrustTarget
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Term of Agreement + References No definition available. + Details
Name: | wtm_TermofAgreement
Namespace Prefix: | wtm_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

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

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CommonStockMember
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_LongtermDebtTypeAxis=wtm_SeriesBBAMSurplusNoteMember
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: | srt_CounterpartyNameAxis=wtm_BamMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

X
- Details
Name: | srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CounterpartyNameAxis=wtm_FidusReLtd.Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=wtm_FidusReLtd.Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |