XML 163 R64.htm IDEA: XBRL DOCUMENT
v3.22.1

Investment Securities - Hedge Funds and Private Equity Funds (Details) $ in Millions | Mar. 31, 2022 USD ($) fund | Dec. 31, 2021 USD ($)
--- | --- | ---
Private equity funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Number of investments | 15 |
Hedge funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Number of investments | 1 |
Hedge and private equity funds included in other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Largest single fund investment | $ | $ 33.9 | $ 31.3

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: | 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: |