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Investment Securities (Non-agency Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Investment securities |  |  |
Fair value investments | $ 5,991.4 us-gaap_InvestmentsFairValueDisclosure | $ 6,766.5 us-gaap_InvestmentsFairValueDisclosure | $ 6,950.7 us-gaap_InvestmentsFairValueDisclosure
Residential |  |  |
Investment securities |  |  |
Fair value investments | 131.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 125.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |
Commercial |  |  |
Investment securities |  |  |
Fair value investments | 236.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 282.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 368.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember | 408.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember |
Securities Issued in 2004 [Member] | Residential |  |  |
Investment securities |  |  |
Fair value investments | 14.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2004Member |  |
Securities Issued in 2004 [Member] | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2004Member |  |
Securities Issued in 2004 [Member] | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 14.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2004Member |  |
2005 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 14.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2005Member |  |
2005 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2005Member |  |
2005 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 14.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2005Member |  |
2006 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 9.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2006Member |  |
2006 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 8.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2006Member |  |
2006 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 18.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2006Member |  |
2007 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2007Member |  |
2007 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2007Member |  |
2007 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2007Member |  |
2008 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 16.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2008Member |  |
2008 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2008Member |  |
2008 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 16.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2008Member |  |
2009 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2009Member |  |
2009 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2009Member |  |
2009 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2009Member |  |
2010 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 14.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2010Member |  |
2010 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 11.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2010Member |  |
2010 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 26.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2010Member |  |
2011 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 21.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2011Member |  |
2011 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2011Member |  |
2011 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 21.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2011Member |  |
2012 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2012Member |  |
2012 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 20.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2012Member |  |
2012 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 20.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2012Member |  |
2013 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 29.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2013Member |  |
2013 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 79.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2013Member |  |
2013 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | 109.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2013Member |  |
2014 | Residential |  |  |
Investment securities |  |  |
Fair value investments | 10.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2014Member |  |
2014 | Commercial |  |  |
Investment securities |  |  |
Fair value investments | 116.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2014Member |  |
2014 | Total Non-agency |  |  |
Investment securities |  |  |
Fair value investments | $ 126.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2014Member |  |

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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