XML 175 R76.htm IDEA: XBRL DOCUMENT
v3.22.1

Third-Party Reinsurance - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2021 | Mar. 31, 2022 | Mar. 31, 2021 | Dec. 31, 2020
Reinsurance [Line Items] |  |  |  |
Reinsurance recoverables on unpaid losses |  | $ 396.0 |  |
Reinsurance recoverables on paid losses |  | 16.7 |  |
Loss and loss adjustment expense reserves | $ 428.9 | 396.0 | $ 447.5 | $ 433.4
TPC Providers |  |  |  |
Reinsurance [Line Items] |  |  |  |
Loss and loss adjustment expense reserves | 276.8 | 207.3 | $ 331.0 | $ 319.2
Ark |  |  |  |
Reinsurance [Line Items] |  |  |  |
Reinsurance recoverables | $ 448.4 | 412.7 |  |
Ark | Third Point Re |  |  |  |
Reinsurance [Line Items] |  |  |  |
Annual crediting rate | 3.00% |  |  |
Decrease in investment | $ 31.7 |  |  |
Carrying value of investment |  | $ 20.6 |  |

X
- Definition Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 5D -Subparagraph (SX 210.12-13D(Column C)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=SL120429264-123010 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column C)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010 + Details
Name: | us-gaap_InvestmentOwnedAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Rate of interest credited to policyholder deposits. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=121643868&loc=d3e14931-158439 + Details
Name: | us-gaap_LiabilityForPolicyholderContractDepositsInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Sum of adjustments to the estimated reserve for unpaid claims and claims adjustment expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
Name: | us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseOpeningBalanceAdjustments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRecoverablesGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount before valuation allowance of reinsurance recoverables on paid losses. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387 + Details
Name: | us-gaap_ReinsuranceRecoverablesOnPaidLossesGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount before valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, and for amounts expected based upon statistical projections and other measures of incurred losses and loss settlement expenses which have not yet been reported to the ceding insurer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489 + Details
Name: | us-gaap_ReinsuranceRecoverablesOnUnpaidLossesGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Investment Owned, Increase (Decrease) In Value + References No definition available. + Details
Name: | wtm_InvestmentOwnedIncreaseDecreaseInValue
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Reinsurance [Line Items] + References No definition available. + Details
Name: | wtm_ReinsuranceLineItems
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | dei_LegalEntityAxis=wtm_ArkInsuranceHoldingsLimitedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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