XML 57 R47.htm IDEA: XBRL DOCUMENT
v3.23.1

Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized investment gains (losses) | $ 46.7 | $ 17.4
Net unrealized investment gains (losses) | 151.4 | (9.1)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 85.2 | 18.8
Gain (loss) on investments, exchange rate effect | 5.4 | 2.6
Investments Held |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized and unrealized investment gains (losses) | 198.1 | 10.4
Fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized investment gains (losses) | (0.1) | (1.1)
Net unrealized investment gains (losses) | 29.7 | (79.5)
Short-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized investment gains (losses) | (0.4) | (0.2)
Net unrealized investment gains (losses) | 1.2 | 0.2
Common equity securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) | 29.4 | 1.0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 29.4 | 1.0
Marketing Technology (MediaAlpha) |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) | 85.2 | 18.8
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 85.2 | 18.8
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net realized investment gains (losses) | 47.2 | 18.7
Net unrealized investment gains (losses) | 5.9 | 50.4
Net realized and unrealized investment (losses) gains |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 198.1 | 8.3
TPC Providers |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 0.0 | 2.1
Fixed maturity and short-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 30.4 | (80.6)
Net realized and unrealized investment gains (losses) | 1.8 | (1.0)
Fixed maturity and short-term investments | Investments Held |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 28.6 | (79.6)
Common equity securities and investment in MediaAlpha |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | 114.6 | 19.8
Net realized and unrealized investment gains (losses) | 0.0 | 0.0
Common equity securities and investment in MediaAlpha | Investments Held |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period | $ 114.6 | $ 19.8

X
- Definition Amount of realized and unrealized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27405-111563 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227 Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585 + 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 -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 + 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 No definition available. + Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
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 -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585 + Details
Name: | us-gaap_UnrealizedGainLossOnInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Gain (Loss) On Investments, Exchange Rate Effect + References No definition available. + Details
Name: | wtm_GainLossOnInvestmentsExchangeRateEffect
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_InvestmentTypeAxis=wtm_InvestmentsHeldMember
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=us-gaap_ShortTermInvestmentsMember
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=wtm_MediaAlphaMember
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_TotalInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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