XML 67 R57.htm IDEA: XBRL DOCUMENT
v3.22.2

Investment Securities - Investment Holding Common Equities/ Other Investments/ MediaAlpha (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2022 | Dec. 31, 2021
Common equity securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | $ 274.3 | $ 236.3
Gross Unrealized Gains | 20.9 | 16.1
Gross Unrealized Losses | (6.7) | 0.0
Net Foreign Currency Gains (Losses) | (9.9) | (1.3)
Common equity securities, at fair value | 278.6 | 251.1
Marketing Technology (MediaAlpha) |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 0.0 | 0.0
Gross Unrealized Gains | 166.9 | 261.6
Gross Unrealized Losses | 0.0 | 0.0
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Common equity securities, at fair value | 166.9 | 261.6
Other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 1,264.2 | 1,186.7
Gross Unrealized Gains | 295.6 | 239.0
Gross Unrealized Losses | (81.1) | (44.1)
Net Foreign Currency Gains (Losses) | (10.0) | (3.8)
Common equity securities, at fair value | $ 1,468.7 | $ 1,377.8

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (f) -URI https://asc.fasb.org/extlink&oid=124098289&loc=d3e6676-107765 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI https://asc.fasb.org/extlink&oid=123594786&loc=SL75136599-209740 + Details
Name: | us-gaap_EquitySecuritiesFvNi
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. + References No definition available. + Details
Name: | us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of unrealized gain on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI https://asc.fasb.org/extlink&oid=126980263&loc=SL75117546-209714 + Details
Name: | us-gaap_EquitySecuritiesFvNiUnrealizedGain
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI https://asc.fasb.org/extlink&oid=126980263&loc=SL75117546-209714 + Details
Name: | us-gaap_EquitySecuritiesFvNiUnrealizedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 Equity Securities, FV-NI, Net Unrealized Foreign Currency Gain (Loss) + References No definition available. + Details
Name: | wtm_EquitySecuritiesFVNINetUnrealizedForeignCurrencyGainLoss
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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=wtm_OtherLongTermInvestmentsExcludingEquityMethodInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |