Contingencies (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	1 Months Ended
Sep. 30, 2014 Esurance	Jun. 30, 2007 Tribune Company Litigation Pending Litigation
Loss Contingencies [Line Items]
Warranty agreement period	18 months
Non-solicitation agreement period	3 years
Percentage of fluctuation in loss reserves of transferred subsidiaries retained	90.00%
Proceeds from Sale, Maturity and Collection of Investments		$ 39.0

Percentage of favorable or unfavorable loss reserve development in the Transferred Subsidiaries retained by the entity.

No definition available.

Name:	wtm_PercentageOfFluctuationInLossReservesOfTransferredSubsidiariesRetained
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant