XML 70 R54.htm IDEA: XBRL DOCUMENT
v3.22.4

Basis of Presentation and Significant Accounting Policies - Reportable Segments (Details) $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jan. 01, 2021 | Dec. 31, 2022 segment | Dec. 31, 2021 | Dec. 31, 2012 USD ($)
Basis of Presentation |  |  |  |
Number of operating segments | segment |  | 3 |  |
Ark |  |  |  |
Basis of Presentation |  |  |  |
Percent of shares outstanding | 12.50% | 12.50% |  |
HG Global |  |  |  |
Basis of Presentation |  |  |  |
Percentage of par value of policy reinsured |  | 15.00% |  |
Surplus notes | $ |  |  |  | $ 503.0
HG Global | Preferred stocks |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) |  | 96.90% | 96.90% |
HG Global | Common equity securities |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) |  | 88.40% | 88.40% |
Ark | Ark |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) |  | 72.00% | 72.00% |
Fully diluted ownership interest (as a percent) |  | 63.00% | 63.00% |
NSM |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) |  |  | 96.50% |
Fully diluted ownership interest (as a percent) |  |  | 87.30% |
Kudu |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) |  | 89.30% | 99.30% |
Fully diluted ownership interest (as a percent) |  | 76.10% | 84.70% |

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 https://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 18 -URI https://asc.fasb.org/extlink&oid=126901519&loc=d3e8672-108599 + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum number of shares that may be issued in accordance with the plan as a proportion of outstanding capital stock. + References No definition available. + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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 https://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
- References No definition available. + Details
Name: | wtm_BasisOfPresentationLineItems
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Equity Method Investment, Fully Diluted Ownership Percentage + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentFullyDilutedOwnershipPercentage
Namespace Prefix: | wtm_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Investment, Fully Diluted Ownership Percentage + References No definition available. + Details
Name: | wtm_InvestmentFullyDilutedOwnershipPercentage
Namespace Prefix: | wtm_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage of par value of policy reinsured. + References No definition available. + Details
Name: | wtm_PercentageOfParValueOfPolicyReinsured
Namespace Prefix: | wtm_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=wtm_ArkInsuranceHoldingsLimitedMember
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: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=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: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=wtm_ArkInsuranceHoldingsLimitedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=wtm_NSMInsuranceHoldCoLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=wtm_KuduInvestmentManagementLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |