XML 101 R76.htm IDEA: XBRL DOCUMENT
v3.25.4

Investment Securities - Investments In Private Equity Funds Subject to Lock-Up Periods (Details) - USD ($) $ in Millions | Dec. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |
1 – 3 years | $ 57.2 |
3 – 5 years | 8.0 |
5 – 10 years | 161.7 |
>10 years | 24.3 |
Total | 2,958.5 | $ 2,150.2
Private equity funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Total | $ 251.2 |

X
- Definition Amount of long-term investments classified as other. + 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)(f)) -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(12)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 3: 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)(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 + Details
Name: | us-gaap_OtherLongTermInvestments
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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6B + Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Other Long-term Investments, Remaining Period, Five to Ten Years + References No definition available. + Details
Name: | wtm_OtherLongTermInvestmentsRemainingPeriodFiveToTenYears
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Other Long-term Investments, Remaining Period, Greater than 10 Years + References No definition available. + Details
Name: | wtm_OtherLongTermInvestmentsRemainingPeriodGreaterThan10Years
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Other Long-term Investments, Remaining Period, One to Three Years + References No definition available. + Details
Name: | wtm_OtherLongTermInvestmentsRemainingPeriodOneToThreeYears
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Other Long-term Investments, Remaining Period, Three to Five Years + References No definition available. + Details
Name: | wtm_OtherLongTermInvestmentsRemainingPeriodThreeToFiveYears
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |