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

X

- 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

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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

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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



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-Publisher SEC

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