Retirement and Postretirement Plans (Details)	12 Months Ended
Dec. 31, 2011 Y
Retirement and Postretirement Plans
Period of service after which plan gets vested (in years)	5

Represents the period over which plan gets vested to the employee under the defined benefit plan.

No definition available.

Name:	wtm_DefinedBenefitPlanVestingPeriod
Namespace Prefix:	wtm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration