XML 98 R63.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Investment Securities (Non-agency Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 5,957.2 us-gaap_InvestmentsFairValueDisclosure | $ 5,991.4 us-gaap_InvestmentsFairValueDisclosure | $ 6,736.7 us-gaap_InvestmentsFairValueDisclosure | $ 6,766.5 us-gaap_InvestmentsFairValueDisclosure
Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 165.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |  |
Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 126.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |  |
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 194.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 131.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |
Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 292.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 236.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  |
Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 486.5 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember | 368.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember |  |
Securities Issued in 2004 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 35.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2004Member |  |  |
Securities Issued in 2004 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2004Member |  |  |
Securities Issued in 2004 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 35.0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2004Member |  |  |
Securities Issued in 2005 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 13.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2005Member |  |  |
Securities Issued in 2005 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2005Member |  |  |
Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 13.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2005Member |  |  |
Securities Issued In 2006 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 9.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2006Member |  |  |
Securities Issued In 2006 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 8.4 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2006Member |  |  |
Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 17.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2006Member |  |  |
Securities Issued In 2007 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2007Member |  |  |
Securities Issued In 2007 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2007Member |  |  |
Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2007Member |  |  |
Securities Issued In 2008 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 15.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2008Member |  |  |
Securities Issued In 2008 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2008Member |  |  |
Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 15.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2008Member |  |  |
Securities Issued In 2009 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2009Member |  |  |
Securities Issued In 2009 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2009Member |  |  |
Securities Issued In 2009 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2009Member |  |  |
Securities Issued In 2010 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 57.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2010Member |  |  |
Securities Issued In 2010 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2010Member |  |  |
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 68.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2010Member |  |  |
Securities Issued In 2011 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 20.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2011Member |  |  |
Securities Issued In 2011 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2011Member |  |  |
Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 20.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2011Member |  |  |
Securities Issued in 2012 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2012Member |  |  |
Securities Issued in 2012 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 26.1 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2012Member |  |  |
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 30.7 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2012Member |  |  |
Securities Issued in 2013 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 28.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2013Member |  |  |
Securities Issued in 2013 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 88.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2013Member |  |  |
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 116.8 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedIn2013Member |  |  |
Securities Issued in 2014 | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 9.9 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2014Member |  |  |
Securities Issued in 2014 | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 122.3 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2014Member |  |  |
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 132.2 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2014Member |  |  |
Securities Issued in 2015 [Member] | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2015Member |  |  |
Securities Issued in 2015 [Member] | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 35.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2015Member |  |  |
Securities Issued in 2015 [Member] | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 35.6 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / wtm_SecurityIssuanceYearAxis = wtm_SecuritiesIssuedin2015Member |  |  |
Subordinate | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 70.8 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |  |
Subordinate | Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 126.6 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |  |
Subordinate | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Subordinate | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 197.4 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SubordinateMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  |  |
Super Senior | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 18.6 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |  |
Super Senior | Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |  |
Super Senior | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 49.3 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Super Senior | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 18.6 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SuperSeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  |  |
Senior | Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 76.2 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFixedRateMember |  |  |
Senior | Commercial Mortgage Backed Securities Floating Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_CommercialMortgageBackedSecuritiesFloatingRateMember |  |  |
Senior | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 145.0 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  |  |
Senior | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 76.2 us-gaap_InvestmentsFairValueDisclosure / wtm_InvestmentByTrancheLevelsAxis = wtm_SeniorMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  |  |

X
- 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
Name: | us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=wtm_CommercialMortgageBackedSecuritiesFixedRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=wtm_CommercialMortgageBackedSecuritiesFloatingRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_InvestmentByTrancheLevelsAxis=wtm_SeniorMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_InvestmentByTrancheLevelsAxis=wtm_SubordinateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_InvestmentByTrancheLevelsAxis=wtm_SuperSeniorMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedin2004Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2005Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2006Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2007Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2008Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2009Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2010Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2011Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2012Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedIn2013Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedin2014Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wtm_SecurityIssuanceYearAxis=wtm_SecuritiesIssuedin2015Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |