XML 72 R68.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Derivatives (Details) In Billions, unless otherwise specified | Sep. 30, 2014 USD ($) | Sep. 30, 2014 JPY (¥) | Dec. 31, 2013 USD ($) | Dec. 31, 2013 JPY (¥) | Mar. 01, 2007 USD ($) | Mar. 01, 2007 JPY (¥) | Sep. 01, 2006 USD ($) | Sep. 01, 2006 JPY (¥)
--- | --- | --- | --- | --- | --- | --- | --- | ---
Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |  |  |  |  |  |  |
Notional amount | $ 1.4 | ¥ 156.8 | $ 1.9 | ¥ 203.6 | $ 0.5 | ¥ 56.0 | $ 1.7 | ¥ 200.0
Collective account values of underlying variable annuities as a percentage of the guarantee value | 107.00% | 107.00% | 104.00% | 104.00% |  |  |  |

X
- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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