XML 110 R89.htm IDEA: XBRL DOCUMENT
v3.25.1

Segment Information - Revenues (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2025 | Mar. 31, 2024
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | $ 395.0 | $ 319.0
Commission and fee revenues | 48.1 | 25.5
Other revenues | 13.6 | 14.3
Total | 456.7 | 358.8
UNITED STATES |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 14.9 | 1.3
Commission and fee revenues | 44.2 | 21.9
Other revenues | 13.6 | 14.3
Total | 72.7 | 37.5
UNITED KINGDOM |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 215.5 | 177.4
Commission and fee revenues | 0.0 | 0.0
Other revenues | 0.0 | 0.0
Total | 215.5 | 177.4
BERMUDA |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 164.6 | 140.3
Commission and fee revenues | 0.0 | 0.0
Other revenues | 0.0 | 0.0
Total | 164.6 | 140.3
Other country |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 0.0 | 0.0
Commission and fee revenues | 3.9 | 3.6
Other revenues | 0.0 | 0.0
Total | $ 3.9 | $ 3.6

X
- Definition Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(d)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1 + Details
Name: | us-gaap_InsuranceCommissionsAndFees
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477965/944-235-S99-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479032/944-605-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -SubTopic 220 -Topic 942 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -SubTopic 220 -Topic 944 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1 + Details
Name: | us-gaap_PremiumsEarnedNet
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_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Other Income, Adjusted + References No definition available. + Details
Name: | wtm_OtherIncomeAdjusted
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Revenues From External Customers + References No definition available. + Details
Name: | wtm_RevenuesFromExternalCustomers
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_GeographicDistributionAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_GeographicDistributionAxis=country_GB
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_GeographicDistributionAxis=country_BM
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_GeographicDistributionAxis=wtm_OtherCountryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |