Summary of Significant Accounting Policies (Investment Securities) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Percentage of investments recorded at fair value	94.00%	94.00%
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	$ 1.0

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