XML 85 R68.htm IDEA: XBRL DOCUMENT
v3.25.0.1

Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations |  |
Cost or Amortized Cost |  |
Due in one year or less | $ 205.8 | $ 319.9
Due after one year through five years | 1,494.5 | 845.0
Due after five years through ten years | 206.1 | 303.4
Due after ten years | 25.3 | 101.7
Carrying Value |  |
Due in one year or less | 203.9 | 315.0
Due after one year through five years | 1,478.7 | 813.9
Due after five years through ten years | 193.2 | 287.0
Due after ten years | 25.2 | 94.9
Mortgage and asset-backed securities and collateralized loan obligations |  |
Cost or Amortized Cost |  |
Mortgage and asset-backed securities and collateralized loan obligations | 637.5 | 628.4
Carrying Value |  |
Mortgage and asset-backed securities and collateralized loan obligations | 610.6 | 598.5
Fixed maturity investments |  |
Cost or Amortized Cost |  |
Total fixed maturity investments | 2,569.2 | 2,198.4
Carrying Value |  |
Total fixed maturity investments | $ 2,511.6 | $ 2,109.3

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

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

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481830/320-10-45-1 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483467/210-10-45-1 + Details
Name: | us-gaap_TradingSecuritiesDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of investments in debt securities classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 + Details
Name: | us-gaap_TradingSecuritiesDebtAmortizedCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingAmortizedCostAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDate
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 10 + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingAmortizedCostAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear10
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year 5 through 10 + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingAmortizedCostAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear5Through10
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, after Year One through Five + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingAmortizedCostAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date, Year One + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingAmortizedCostAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateYearOne
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingFairValueMaturityAllocatedAndSingleMaturityDate
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10 + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingFairValueMaturityAllocatedAndSingleMaturityDateAfterYear10
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10 + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingFairValueMaturityAllocatedAndSingleMaturityDateAfterYear5Through10
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingFairValueMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Debt Securities, Trading, Fair Value, Maturity, Allocated And Single Maturity Date, Year One + References No definition available. + Details
Name: | wtm_DebtSecuritiesTradingFairValueMaturityAllocatedAndSingleMaturityDateYearOne
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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

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