XML 71 R61.htm IDEA: XBRL DOCUMENT
v3.22.2.2

Investment Securities - Mortgage-backed Asset Securities Table (Details) - USD ($) $ in Millions | Sep. 30, 2022 | Dec. 31, 2021
--- | --- | ---
Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | $ 217.9 | $ 256.5
Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 22.8 | 21.1
Total mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 240.7 | 277.6
Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 157.3 | 135.0
Total mortgage and asset-backed securities and collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 398.0 | 412.6
Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 217.9 | 256.5
Level 2 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 22.8 | 21.1
Level 2 | Total mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 240.7 | 277.6
Level 2 | Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 157.3 | 135.0
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 398.0 | 412.6
Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Level 3 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Level 3 | Total mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Level 3 | Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
FNMA | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 114.1 | 125.4
FNMA | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 114.1 | 125.4
FNMA | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
FHLMC | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 74.5 | 90.5
FHLMC | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 74.5 | 90.5
FHLMC | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
GNMA | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 28.9 | 40.1
GNMA | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 28.9 | 40.1
GNMA | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Total agency | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 217.5 | 256.0
Total agency | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 217.5 | 256.0
Total agency | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Non-agency: Residential | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.4 | 0.5
Non-agency: Residential | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.4 | 0.5
Non-agency: Residential | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Total non-agency | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.4 | 0.5
Total non-agency | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.4 | 0.5
Total non-agency | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Credit card receivables | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 11.9 | 12.3
Credit card receivables | Level 2 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 11.9 | 12.3
Credit card receivables | Level 3 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | 0.0
Vehicle receivables | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 10.9 | 8.8
Vehicle receivables | Level 2 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 10.9 | 8.8
Vehicle receivables | Level 3 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | 0.0 | $ 0.0
Collateralized mortgage obligations | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fair value investments | $ 157.3 |

X
- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258 + Details
Name: | us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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
- 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_AssetBackedSecuritiesMember
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_FinancialInstrumentAxis=wtm_MortgageAndAssetBackedSecuritiesAndCollateralizedLoanObligationsMember
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_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_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
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_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
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=us-gaap_CollateralizedAutoLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |