XML 15 R70.htm IDEA: XBRL DOCUMENT
v2.4.0.6

Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012
--- | --- | --- | ---
Credit risk on derivative instruments |  |  |
Collateral cash and investments | $ 229.8 | $ 393.6 | $ 390.9
Cash |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | 188.5 | 249.8 | 360.1
Short-term investments |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | 15.6 | 5.1 | 1.5
Fixed maturity investments. |  |  |
Credit risk on derivative instruments |  |  |
Derivative collateral right to reclaim | $ 25.7 | $ 138.7 | $ 29.3

X
- 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