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Investment Securities (Fair Value Measurements) (Details) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Percentage of investments recorded at fair value	89.00%	94.00%

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- Definition

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

+ References

No definition available.

+ Details

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