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Investment Securities (Net Investment Income) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Investment income: |  |  |
Total investment income | $ 125.6 us-gaap_InterestAndDividendIncomeOperating | $ 130.6 us-gaap_InterestAndDividendIncomeOperating | $ 167.0 us-gaap_InterestAndDividendIncomeOperating
Third-party investment expenses | (20.6)us-gaap_InvestmentIncomeInvestmentExpense | (19.7)us-gaap_InvestmentIncomeInvestmentExpense | (13.4)us-gaap_InvestmentIncomeInvestmentExpense
Net investment income, pre-tax | 105.0 us-gaap_NetInvestmentIncome | 110.9 us-gaap_NetInvestmentIncome | 153.6 us-gaap_NetInvestmentIncome
Fixed maturity investments |  |  |
Investment income: |  |  |
Total investment income | 98.9 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 100.5 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 132.0 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Short-term investments |  |  |
Investment income: |  |  |
Total investment income | 1.5 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 3.4 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember | 3.1 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ShortTermInvestmentsMember
Common Stock |  |  |
Investment income: |  |  |
Total investment income | 21.0 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 20.5 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 22.2 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Convertible fixed maturity and preferred investments |  |  |
Investment income: |  |  |
Total investment income | 2.5 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 2.9 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 6.0 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments |  |  |
Investment income: |  |  |
Total investment income | 1.9 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 3.0 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | 3.1 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember
Interest on funds held under reinsurance treaties |  |  |
Investment income: |  |  |
Total investment income | $ (0.2)us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_FundsHeldUnderReinsuranceAgreementsMember | $ 0.3 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_FundsHeldUnderReinsuranceAgreementsMember | $ 0.6 us-gaap_InterestAndDividendIncomeOperating / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_FundsHeldUnderReinsuranceAgreementsMember

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- Definition Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.1-5) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1, 2 , 3, 4, 5 -Article 9 + Details
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- Definition Amount of expenses related to the generation of investment income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 16 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 2 -Article 7 + Details
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