Significant Transactions (Sale of Esurance) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	1 Months Ended
Dec. 31, 2011	Oct. 31, 2011 Esurance
Significant Transactions
Consideration in excess of tangible book value		$ 700
Gain on sale of business in discontinued operations	$ 700.0	$ 677.5

Consideration in excess of tangible book value for the sale of a business

No definition available.

Name:	wtm_ConsiderationInExcessOfTangibleBookValueForSaleOfBusiness
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration