Investment Securities (Fair Value Measurements) (Details) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Percentage of investments recorded at fair value	93.00%	94.00%
Maximum percentage of price difference provided by pricing services	5.00%
Maximum price difference provided by pricing services	$ 1.0

Represents the maximum percentage of price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered.

No definition available.

Name:	wtm_MaximumPercentageDifferencesBetweenPricesProvidedByPricingServicesConsideredOutliers
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration