Basis of Presentation and Significant Accounting Policies - Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Percentage of investments recorded at fair value	73.00%	71.00%
Variance threshold for outliers	$ 0.5
Variance threshold for outliers (as a percent)	5.00%

Represents the percentage of the total investment portfolio recorded at fair value based upon quoted market prices or other observable inputs.

No definition available.

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