Summary of Significant Accounting Policies (Investment Securities) (Details) $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($) security	Dec. 31, 2014
Accounting Policies [Abstract]
Percentage of investments recorded at fair value	91.00%	88.00%
Minimum percentage of variation expected	5.00%
Minimum variation expected \| $	$ 1.0
Securities outside variance threshold \| security	8

Represents the minimum percentage of variation identified during sales backtesting of prices of investments provided by pricing services from the historical sales price, required for treating the prices provided by pricing services as outliers.

No definition available.

Name:	wtm_MinimumPercentageOfVariationFromExpectedPriceRequiredToTreatPricesOfInvestmentsProvidedByPricingServicesAsOutliers
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant