Summary of Significant Accounting Policies (Insurance and Reinsurance Operations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Discount rate on workers compensation loss and loss adjustment reserves	3.50%	4.50%
Discount amount on workers compensation loss and loss adjustment reserves	$ 4.6	$ 108.3
OneBeacon Runoff
Discount amount on workers compensation loss and loss adjustment reserves	$ 77.9

Represents the average discount rate used to discount certain of the long-term workers compensation loss and LAE reserves, which is determined based on the various assumptions including consideration of when the claims will be settled.

No definition available.

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