XML 91 R66.htm IDEA: XBRL DOCUMENT
v2.4.0.6

Derivatives (Details) In Billions, unless otherwise specified | Mar. 31, 2013 USD ($) | Mar. 31, 2013 JPY (¥) | Dec. 31, 2012 USD ($) | Dec. 31, 2012 JPY (¥)
--- | --- | --- | --- | ---
Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |  |  |
Total guarantee value | $ 2.4 | ¥ 228.5 | $ 2.7 | ¥ 230.0
Collective account values of underlying variable annuities as a percentage of the guarantee value | 94.00% | 94.00% | 87.00% | 87.00%

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

X
- Definition Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability. + References No definition available. + Details
Name: | wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Total notional account value guaranteed with certain variable annuities. + References No definition available. + Details
Name: | wtm_VariableAnnuityNotionalGuaranteeValue
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant