XML 58 R47.htm IDEA: XBRL DOCUMENT
v3.24.3

Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) (1) | $ 223.8 | $ (58.3) | $ 390.4 | $ 151.2
MediaAlpha |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) (1) | 88.2 | (46.8) | 159.7 | (38.9)
Net realized and unrealized investment (losses) gains |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized investment gains (losses) | 4.3 | 1.3 | 119.2 | 60.7
Net unrealized investment gains (losses) | 219.5 | (59.6) | 271.2 | 90.5
Fixed maturity investments |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized investment gains (losses) | (0.5) | (0.3) | (9.2) | (1.7)
Net unrealized investment gains (losses) | 56.9 | (27.9) | 49.7 | (9.7)
Short-term investments |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized investment gains (losses) | 0.0 | (0.2) | (0.3) | (0.3)
Net unrealized investment gains (losses) | 0.5 | (0.6) | (0.5) | 0.7
Common equity securities |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized investment gains (losses) | 0.0 | 1.2 | 3.5 | 13.8
Net unrealized investment gains (losses) | 24.7 | (18.5) | 63.6 | 29.8
Net realized and unrealized investment gains (losses) (1) | 24.7 | (17.3) | 67.1 | 43.6
Equity Method Investments | MediaAlpha |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized investment gains (losses) | 0.0 | 0.0 | 91.2 | 0.0
Net unrealized investment gains (losses) | 88.2 | (46.8) | 68.5 | (38.9)
Net realized and unrealized investment gains (losses) (1) | 88.2 | (46.8) | 159.7 | (38.9)
Other long-term investments |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized investment gains (losses) | 4.8 | 0.6 | 34.0 | 48.9
Net unrealized investment gains (losses) | 49.2 | 34.2 | 89.9 | 108.6
Fixed maturity and short-term investments |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) (1) | 56.9 | (29.0) | 39.7 | (11.0)
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 1.2 | 2.5 | 0.8 | 4.7
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 55.7 | (31.5) | 38.9 | (15.7)
Common equity securities and investment in MediaAlpha |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net realized and unrealized investment gains (losses) (1) | 112.9 | (64.1) | 226.8 | 4.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 0.0 | (0.1) | 35.4 | 3.7
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 112.9 | (64.0) | 191.4 | 1.0
Total Investments After TPC Providers - Investments |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Gain (loss) on investments, exchange rate effect | $ 18.9 | $ (12.8) | $ 7.1 | $ (1.9)

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: |