XML 74 R64.htm IDEA: XBRL DOCUMENT
v3.22.2.2

Investment Securities - Hedge Funds and Private Equity Funds (Details) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2022 USD ($) fund | Sep. 30, 2021 USD ($) | Dec. 31, 2021 USD ($)
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Investments in trading securities | $ 97.9 | $ 141.6 |
TIG Advisors, TK Partners, Third Eye Capital Management, Douglass Winthrop Advisors, Granahan Investment Management and Radcliffe Capital Management | Kudu |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Investments in trading securities |  |  | $ 223.4
Private equity funds |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Number of investments | fund | 16 |  |
Hedge funds |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Number of investments | fund | 2 |  |
Total private equity funds and hedge funds included in other long-term investments |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Largest single fund investment | $ 49.3 |  | $ 31.3

X
- Definition Amount of increase (decrease) of investment in debt and equity securities, measured at fair value with changes in fair value recognized in net income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 19 -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3444-108585 + Details
Name: | us-gaap_IncreaseDecreaseInTradingSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Investments + References No definition available. + Details
Name: | wtm_NumberOfInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The largest investment which the entity has made in a single fund. + References No definition available. + Details
Name: | wtm_OtherLongTermInvestmentsLargestSingleInvestment
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | wtm_Investment1Axis=wtm_TIGAdvisorsTKPartnersThirdEyeCapitalManagementDouglassWinthropAdvisorsGranahanInvestmentManagementAndRadcliffeCapitalManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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