XML 180 R82.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Investment Securities (Investments Held on Deposit or as Collateral) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Investment securities |  |
Investments held in trusts | $ 171.5 wtm_InvestmentsHeldInTrusts | $ 170.4 wtm_InvestmentsHeldInTrusts
Assets held by insurance regulators | 130.3 us-gaap_AssetsHeldByInsuranceRegulators | 290.7 us-gaap_AssetsHeldByInsuranceRegulators
Surety Product Line [Member] | OneBeacon |  |
Investment securities |  |
Assets Held in Collateral | 81.0 wtm_AssetsHeldinCollateral / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_SuretySegmentMember | 63.3 wtm_AssetsHeldinCollateral / dei_LegalEntityAxis = wtm_OnebeaconMember / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_SuretySegmentMember
Fixed maturity investments | White Mountains Life Re |  |
Investment securities |  |
Derivative Collateral Right to Reclaim | 9.5 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 23.2 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Short-term investments | White Mountains Life Re |  |
Investment securities |  |
Derivative Collateral Right to Reclaim | 0 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 2.0 wtm_DerivativeCollateralRightToReclaim / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Interest Rate Cap |  |
Investment securities |  |
Derivative Collateral Right to Reclaim | $ 4.3 wtm_DerivativeCollateralRightToReclaim / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateCapMember |

X
- Definition Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28366721&loc=d3e22002-158495 + Details
Name: | us-gaap_AssetsHeldByInsuranceRegulators
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Cash and investments held in collateral. + References No definition available. + Details
Name: | wtm_AssetsHeldinCollateral
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of collateral held against amounts due to counterparties for over the counter derivative instruments against net derivative instrument positions. + References No definition available. + Details
Name: | wtm_DerivativeCollateralRightToReclaim
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Investments Held In Trusts + References No definition available. + Details
Name: | wtm_InvestmentsHeldInTrusts
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateCapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

X
- Details
Name: | us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_SuretySegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |