XML 28 R76.htm IDEA: XBRL DOCUMENT
v3.3.0.814

Derivatives Additional Collateral (Details) - White Mountains Life Re - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014 | Sep. 30, 2014
--- | --- | --- | ---
Credit risk on derivative instruments |  |  |
Collateral cash and investments | $ 16.8 | $ 33.2 | $ 34.2
Cash |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | 16.8 | 23.7 | 23.8
Fixed maturity investments. |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | $ 0.0 | $ 9.5 | $ 10.4

X
- References No definition available. + Details
Name: | us-gaap_DerivativeCreditRiskRelatedContingentFeaturesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents cash and short-term investments held and posted as collateral to reinsurance counterparties. + References No definition available. + Details
Name: | wtm_CollateralCashAndInvestments
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
- Details
Name: | dei_LegalEntityAxis=wtm_WhiteMountainsLifeReMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_CashAndCashEquivalentsMember
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: |