XML 70 R49.htm IDEA: XBRL DOCUMENT
v3.25.1

Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2025 | Mar. 31, 2024
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) | $ 50.0 | $ 226.8
MediaAlpha |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) | (36.6) | 210.7
Net realized and unrealized investment (losses) gains |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | 40.3 | (5.7)
Net unrealized investment gains (losses) | 9.7 | 232.5
Fixed maturity investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | (0.1) | (5.8)
Net unrealized investment gains (losses) | 22.2 | (7.4)
Short-term investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | 0.2 | (0.2)
Net unrealized investment gains (losses) | 0.0 | (1.0)
Common equity securities |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | 34.6 | 0.0
Net unrealized investment gains (losses) | (22.1) | 23.1
Net realized and unrealized investment gains (losses) | 12.5 | 23.1
Equity Method Investments | MediaAlpha |  |
Gain (Loss) on Securities [Line Items] |  |
Net unrealized investment gains (losses) | (36.6) | 210.7
Net realized and unrealized investment gains (losses) | (36.6) | 210.7
Other long-term investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized investment gains (losses) | 5.6 | 0.3
Net unrealized investment gains (losses) | 46.2 | 7.1
Fixed maturity and short-term investments |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) | 22.3 | (14.4)
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 0.5 | (0.7)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 21.8 | (13.7)
Common equity securities and investment in MediaAlpha |  |
Gain (Loss) on Securities [Line Items] |  |
Net realized and unrealized investment gains (losses) | (24.1) | 233.8
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 3.0 | 0.0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | (27.1) | 233.8
Total Investments After TPC Providers - Investments |  |
Gain (Loss) on Securities [Line Items] |  |
Gain (loss) on investments, exchange rate effect | $ 7.2 | $ (11.5)

X
- Definition Amount, after tax, of realized and unrealized gain (loss) from foreign currency transaction. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 946 -SubTopic 205 -Name Accounting Standards Codification -Section 45 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478009/946-205-45-3 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 946 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.6-09(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479134/946-220-S99-3 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 45 -Paragraph 38 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477346/946-830-45-38 + Details
Name: | us-gaap_ForeignCurrencyTransactionGainLossAfterTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of realized and unrealized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(9)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(7)(c)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2 Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-28 + Details
Name: | us-gaap_GainLossOnInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-28 + Details
Name: | us-gaap_GainLossOnSaleOfInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of realized gain (loss) on investment. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(3)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1 + Details
Name: | us-gaap_RealizedInvestmentGainsLosses
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 Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(9)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(7)(c)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-9 + Details
Name: | us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of unrealized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-28 + Details
Name: | us-gaap_UnrealizedGainLossOnInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Net Unrealized Gains (Losses) On Investments + References No definition available. + Details
Name: | wtm_NetUnrealizedGainsLossesOnInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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

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

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

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

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

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

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