XML 156 R120.htm IDEA: XBRL DOCUMENT v2.4.1.9
Derivatives Derivative (Tranzact Interest Rate Swap) (Details) (Details) (Tranzact [Member], USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Oct. 10, 2014
Dec. 31, 2013
Mar. 31, 2014
Tranzact Bank Facility [Member]
       
Derivative [Line Items]        
Line of Credit Facility, Fair Value of Amount Outstanding $ 67.4us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding
/ us-gaap_DebtInstrumentAxis
= wtm_TranzactBankFacilityMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
$ 70.0us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding
/ us-gaap_DebtInstrumentAxis
= wtm_TranzactBankFacilityMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
$ 0us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding
/ us-gaap_DebtInstrumentAxis
= wtm_TranzactBankFacilityMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
 
Line of Credit Facility, Amount Outstanding 68.7us-gaap_LineOfCreditFacilityAmountOutstanding
/ us-gaap_DebtInstrumentAxis
= wtm_TranzactBankFacilityMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
  0us-gaap_LineOfCreditFacilityAmountOutstanding
/ us-gaap_DebtInstrumentAxis
= wtm_TranzactBankFacilityMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
 
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate 4.15575%us-gaap_LongTermDebtPercentageBearingVariableInterestRate
/ us-gaap_DebtInstrumentAxis
= wtm_TranzactBankFacilityMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
     
Minimum | Tranzact Bank Facility [Member]        
Derivative [Line Items]        
LIBOR rate 2.50%wtm_LIBORrate
/ us-gaap_DebtInstrumentAxis
= wtm_TranzactBankFacilityMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Interest Rate Swap [Member]        
Derivative [Line Items]        
Derivative, Fixed Interest Rate 1.34%us-gaap_DerivativeFixedInterestRate
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
     
Derivative, Variable Interest Rate 0.1558%us-gaap_DerivativeVariableInterestRate
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
0.1518%us-gaap_DerivativeVariableInterestRate
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ dei_LegalEntityAxis
= wtm_TranzactMember
   
Derivative, Fair Value, Net       $ 0.1us-gaap_DerivativeFairValueOfDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ dei_LegalEntityAxis
= wtm_TranzactMember