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Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid and unpaid losses $ 507.5us-gaap_ReinsuranceRecoverablesGross $ 453.5us-gaap_ReinsuranceRecoverablesGross
Sirius Group    
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid and unpaid losses 333.6us-gaap_ReinsuranceRecoverablesGross
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		 363.6us-gaap_ReinsuranceRecoverablesGross
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember
Reinsurance Recoverables Allowance 30.2us-gaap_ReinsuranceRecoverablesAllowance
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables 303.4us-gaap_ReinsuranceRecoverables
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance recoverables, percentage of total 100.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
AA | Sirius Group    
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid and unpaid losses 114.6us-gaap_ReinsuranceRecoverablesGross
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables Allowance 1.6us-gaap_ReinsuranceRecoverablesAllowance
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables 113.0us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance recoverables, percentage of total 34.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
A | Sirius Group    
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid and unpaid losses 138.8us-gaap_ReinsuranceRecoverablesGross
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables Allowance 7.1us-gaap_ReinsuranceRecoverablesAllowance
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables 131.7us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance recoverables, percentage of total 42.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
BBB plus | Sirius Group    
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid and unpaid losses 8.9us-gaap_ReinsuranceRecoverablesGross
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables Allowance 0us-gaap_ReinsuranceRecoverablesAllowance
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables 8.9us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance recoverables, percentage of total 3.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBPlusRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
BBB | Sirius Group    
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid and unpaid losses 9.6us-gaap_ReinsuranceRecoverablesGross
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables Allowance 0us-gaap_ReinsuranceRecoverablesAllowance
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables 9.6us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance recoverables, percentage of total 3.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBRatingMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Not rated | Sirius Group    
Reinsurance Retention Policy [Line Items]    
Reinsurance recoverable on paid and unpaid losses 61.7us-gaap_ReinsuranceRecoverablesGross
/ us-gaap_CreditRatingStandardPoorsAxis
= wtm_NotRatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables Allowance 21.5us-gaap_ReinsuranceRecoverablesAllowance
/ us-gaap_CreditRatingStandardPoorsAxis
= wtm_NotRatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance Recoverables $ 40.2us-gaap_ReinsuranceRecoverables
/ us-gaap_CreditRatingStandardPoorsAxis
= wtm_NotRatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember		  
Reinsurance recoverables, percentage of total 18.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= wtm_NotRatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= wtm_SiriusGroupMember