Summary of Significant Accounting Policies (Investment Securities) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Minimum Percentage of Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	5.00%
Minimum Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	$ 0.5
Percentage of investments recorded at fair value	87.00%	94.00%

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