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Third Party Reinsurance (Sirius Group) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid losses $ 29.8us-gaap_ReinsuranceRecoverablesOnPaidLosses $ 23.6us-gaap_ReinsuranceRecoverablesOnPaidLosses
Reinsurance recoverable on unpaid losses 436.9us-gaap_ReinsuranceRecoverablesOnUnpaidLosses 483.9us-gaap_ReinsuranceRecoverablesOnUnpaidLosses
Sirius Group    
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid losses 11.6us-gaap_ReinsuranceRecoverablesOnPaidLosses
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		 11.4us-gaap_ReinsuranceRecoverablesOnPaidLosses
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember
Reinsurance recoverable on unpaid losses $ 295.3us-gaap_ReinsuranceRecoverablesOnUnpaidLosses
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		 $ 322.2us-gaap_ReinsuranceRecoverablesOnUnpaidLosses
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember