XML 65 R54.htm IDEA: XBRL DOCUMENT v3.24.1.u1

Investment Securities - Mortgage and Asset-Backed Securities and Collateralized Loan Obligations (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 696.6	$ 598.5
Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.1	389.4
Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	404.5	366.4
Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	45.6	23.0
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	246.5	209.1
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	696.6	598.5
Level 2 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	450.1	389.4
Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	404.5	366.4
Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	45.6	23.0
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	246.5	209.1
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Total mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Total non-agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
Total non-agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
Total non-agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Total agency | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	404.4	366.2
Total agency | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	404.4	366.2
Total agency | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
FNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	216.7	198.8
FNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	216.7	198.8
FNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
FHLMC | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	147.8	137.2
FHLMC | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	147.8	137.2
FHLMC | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
GNMA | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	39.9	30.2
GNMA | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	39.9	30.2
GNMA | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Non-agency: Residential | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
Non-agency: Residential | Level 2 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.1	0.2
Non-agency: Residential | Level 3 | Mortgage-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Vehicle receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	31.1	17.2
Vehicle receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	31.1	17.2
Vehicle receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Credit card receivables | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.3	3.3
Credit card receivables | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.3	3.3
Credit card receivables | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Other | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	11.2	2.5
Other | Level 2 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	11.2	2.5
Other | Level 3 | Other asset-backed securities:
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 0.0	$ 0.0

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 investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481830/320-10-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483467/210-10-45-1

+ Details

Name:	us-gaap_TradingSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=wtm_MortgageAndAssetBackedSecuritiesAndCollateralizedLoanObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedLoanObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedAutoLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedCreditCardSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=wtm_OtherAssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: