Summary of Significant Accounting Policies (Details) (OneBeacon Ltd.) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2006	Dec. 31, 2011	Dec. 31, 2010
OneBeacon Ltd.
Basis of Presentation
Ownership percentage in outstanding common shares		75.50%	76.00%
Number of common shares sold in an initial public offerings	27.6
Percentage of common shares sold in an initial public offerings	27.60%

Represents the shares issued in the disposition of equity in securities of subsidiaries as a percentage of total shares of the subsidiary.

No definition available.

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