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Fair Value of Financial Instruments (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Debt $ 749.8us-gaap_LongTermDebt $ 746.6us-gaap_LongTermDebt
2012 OBH Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Debt 291.5us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_A2012ObhSeniorNotesMember		 286.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_A2012ObhSeniorNotesMember
SIG Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Debt 438.2us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember		 437.8us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember
SIG Preference Shares    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Debt 261.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SIGPreferenceSharesMember		 260.0us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SIGPreferenceSharesMember
Book Value | 2012 OBH Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Debt 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
Book Value | SIG Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Debt 399.7us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 399.7us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= wtm_SiriusInternationalSeniorNotesMember
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Book Value | SIG Preference Shares    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Debt $ 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