Investment Securities (Fair Value Measurements) (Details) (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Percentage of investments recorded at fair value	92.00%	94.00%

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