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Investment Securities (Hedge Funds and Private Equity Funds) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Other Investments $ 402.6us-gaap_OtherInvestments $ 407.0us-gaap_OtherInvestments
Hedge funds    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Number of investments 12wtm_NumberOfInvestmentsHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember		  
Private equity funds    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Number of investments 31wtm_NumberOfInvestmentsHeld
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PrivateEquityFundsMember		  
Hedge and private equity funds included in other long-term investments    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Largest single fund investment $ 29.4wtm_OtherLongTermInvestmentsLargestSingleInvestment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= wtm_HedgeFundsAndPrivateEquityFundsMember