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Debt and Standby Letter of Credit Facilities (Schedule of Contractual Repayments) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Debt Disclosure [Abstract] |
Due in one year or less | $ 6.0 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths
Due in two to three years | 424.6 us-gaap_LongTermDebtMaturingInYearsTwoAndThree
Due in four to five years | 42.9 us-gaap_LongTermDebtMaturingInYearsFourAndFive
Due after five years | 275.0 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive
Total | $ 748.5 wtm_LongTermDebtMaturitiesRepaymentsOfPrincipalTotal

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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fourth and fifth fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the second and third fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
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- Definition Total amount of long-term debt maturing in future years following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates. + References No definition available. + Details
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