Investment Securities - Fair Value Measurements (Details)	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Percentage of investments recorded at fair value	62.00%	71.00%

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