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Fair Value of Financial Instruments (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes $ 746.6us-gaap_LongTermDebt $ 676.4us-gaap_LongTermDebt
Fair value | 2012 OBH Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 286.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_A2012ObhSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 269.8us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_A2012ObhSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair value | SIG Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 437.8us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 438.1us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair value | SIG Preference Shares    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 260.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SIGPreferenceSharesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 260.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SIGPreferenceSharesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Carrying Value | 2012 OBH Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 274.7us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_A2012ObhSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 274.7us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_A2012ObhSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Value | SIG Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 399.7us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 399.6us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Value | SIG Preference Shares    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes $ 250.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SIGPreferenceSharesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 $ 250.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SIGPreferenceSharesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember