XML 142 R144.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended | 6 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Jun. 20, 2013
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  | $ 321.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |  |  | $ 321.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax | 12.3 us-gaap_IncomeLossFromEquityMethodInvestments | 7.0 us-gaap_IncomeLossFromEquityMethodInvestments | 12.5 us-gaap_IncomeLossFromEquityMethodInvestments | 13.8 us-gaap_IncomeLossFromEquityMethodInvestments | 11.7 us-gaap_IncomeLossFromEquityMethodInvestments | 8.6 us-gaap_IncomeLossFromEquityMethodInvestments | 7.1 us-gaap_IncomeLossFromEquityMethodInvestments | 9.2 us-gaap_IncomeLossFromEquityMethodInvestments | 45.6 us-gaap_IncomeLossFromEquityMethodInvestments | 36.6 us-gaap_IncomeLossFromEquityMethodInvestments | 29.9 us-gaap_IncomeLossFromEquityMethodInvestments |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  | 81.2 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio | (106.4)wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio | 62.8 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio |  |
Carrying value at the end of the period | 414.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |  | 321.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |  | 414.4 wtm_InvestmentsInUnconsolidatedAffiliates | 321.4 wtm_InvestmentsInUnconsolidatedAffiliates |  |  |
Income Tax Expense (Benefit) | (8.7)us-gaap_IncomeTaxExpenseBenefit | 7.8 us-gaap_IncomeTaxExpenseBenefit | 23.9 us-gaap_IncomeTaxExpenseBenefit | 30.3 us-gaap_IncomeTaxExpenseBenefit | 27.4 us-gaap_IncomeTaxExpenseBenefit | 8.2 us-gaap_IncomeTaxExpenseBenefit | (0.6)us-gaap_IncomeTaxExpenseBenefit | 41.6 us-gaap_IncomeTaxExpenseBenefit | 53.3 us-gaap_IncomeTaxExpenseBenefit | 76.6 us-gaap_IncomeTaxExpenseBenefit | (15.7)us-gaap_IncomeTaxExpenseBenefit |  |
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 381.5 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 381.5 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 273.6 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  | 381.5 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | 47.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 37.8 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 32.3 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  | 81.2 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | (106.4)wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 62.8 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |
Dividends received |  |  |  |  |  |  |  |  | (34.1)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | (6.4)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | (4.9)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  | 10.8 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 17.7 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |
Carrying value at the end of the period | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 381.5 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | 273.6 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |
Unrealized gains (losses) from Symetra’s fixed maturity portfolio | 37.6 wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | (43.6)wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |  | 37.6 wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember | (43.6)wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Equity in net unrealized losses increase relating to amortization of common share impairment |  |  |  |  |  |  |  |  |  |  |  | 136.6 wtm_Othercomprehensiveincomeincreaserelatingtoamortizartionofcommonsharebasisdifference / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |
Exercise of warrants |  |  |  |  |  |  |  |  |  | 0 wtm_Exerciseofwarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember |  |  |
Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  | 351.2 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 351.2 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 261.0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  | 351.2 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | 47.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 37.8 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 32.3 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  | 81.2 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | (106.4)wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 62.8 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Dividends received |  |  |  |  |  |  |  |  | (34.1)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | (6.4)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | (4.9)us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  | 0 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 0 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Carrying value at the end of the period | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  | 411.4 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 317.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 351.2 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Loss from dilutive effect resulting from dividend and issuance of restricted shares |  |  |  |  |  |  |  |  | (0.1)wtm_LossFromDilutiveEffectResultingFromDividendAndIssuanceOfRestrictedShares / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 0.2 wtm_LossFromDilutiveEffectResultingFromDividendAndIssuanceOfRestrictedShares / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 1.3 wtm_LossFromDilutiveEffectResultingFromDividendAndIssuanceOfRestrictedShares / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Aggregate value of White Mountains common shares of Symetra at quoted market price | 462.1 wtm_InvestmentsInUnconsolidatedAffiliatesAtQuotedMarketPrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  | 462.1 wtm_InvestmentsInUnconsolidatedAffiliatesAtQuotedMarketPrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |
Share Price | $ 23.05 us-gaap_SharePrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  | $ 23.05 us-gaap_SharePrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |  |
Exercise of warrants |  |  |  |  |  |  |  |  |  | 41.1 wtm_Exerciseofwarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |
Common Stock | White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  | 37.6 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_WhiteMountainsMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | (43.6)wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_WhiteMountainsMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | 62.8 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_WhiteMountainsMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |
Warrants |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 10.8 wtm_GainLossOnWarrants / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember
Warrants | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  | 0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |  |  | 30.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 30.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 12.6 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  | 30.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  | 0 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Dividends received |  |  |  |  |  |  |  |  | 0 us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 us-gaap_EquityMethodInvestmentDividendsOrDistributions / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  | 10.8 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 17.7 wtm_GainLossOnWarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Carrying value at the end of the period | 0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |  |  | 0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |  |  | 0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 0 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember | 30.3 wtm_InvestmentsInUnconsolidatedAffiliates / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Share Price |  |  |  |  |  |  |  |  |  |  | $ 11.49 us-gaap_SharePrice / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |
Exercise of warrants |  |  |  |  |  |  |  |  |  | (41.1)wtm_Exerciseofwarrants / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_WarrantMember |  |  |
equity in earnings of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 3.3 us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember | 2.8 us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember | 2.6 us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  |
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 5.9 us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember | (8.3)us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember | 5.1 us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  |
Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | White Mountains |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | (2.7)us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_WhiteMountainsMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember | 3.2 us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_WhiteMountainsMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember | (5.1)us-gaap_IncomeTaxExpenseBenefit / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_WhiteMountainsMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  |
impairment of equity in earnings [Member] | equity in earnings of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  |  |  | 3.5 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ExtraordinaryItemAxis = wtm_ImpairmentofequityinearningsMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_EquityinearningsofunconsolidatedaffiliatesMember |  |
impairment of net unrealized gains (losses) [Member] | Net unrealized gains (losses) of unconsolidated affiliates [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  | 12.7 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_ExtraordinaryItemAxis = wtm_ImpairmentofnetunrealizedgainslossesMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember | 11.8 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_ExtraordinaryItemAxis = wtm_ImpairmentofnetunrealizedgainslossesMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember | 13.1 wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio / us-gaap_ExtraordinaryItemAxis = wtm_ImpairmentofnetunrealizedgainslossesMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember / us-gaap_StatementEquityComponentsAxis = wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember |  |
Amortization of common share basis difference [Member] | Common Stock | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax |  |  |  |  |  |  |  |  | $ 3.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ExtraordinaryItemAxis = wtm_AmortizationofcommonsharebasisdifferenceMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember | $ 3.0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ExtraordinaryItemAxis = wtm_AmortizationofcommonsharebasisdifferenceMember / us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis = wtm_SymetraMember / us-gaap_InvestmentTypeAxis = us-gaap_CommonStockMember |  |  |

X
- Definition This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Article 5 + Details
Name: | us-gaap_EquityMethodInvestmentDividendsOrDistributions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=16385135&loc=d3e33749-111570 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.12) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 7 + Details
Name: | us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
Name: | us-gaap_IncomeTaxExpenseBenefit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Price of a single share of a number of saleable stocks of a company. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
Name: | us-gaap_SharePrice
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates. + References No definition available. + Details
Name: | wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The net unrealized gains (losses) from Symetra's fixed maturity portfolio. + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentInvesteeUnrealizedGainsLossesPortfolio
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The value of the exercise of warrants. + References No definition available. + Details
Name: | wtm_Exerciseofwarrants
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The increase (decrease) in the fair value of warrants included in other revenue. + References No definition available. + Details
Name: | wtm_GainLossOnWarrants
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Total investments in (A) companies that are accounted for using the equity method and (B) warrants to acquire additional shares of an unconsolidated affiliate that are carried at fair value. + References No definition available. + Details
Name: | wtm_InvestmentsInUnconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the aggregate value of common shares at quoted market price. + References No definition available. + Details
Name: | wtm_InvestmentsInUnconsolidatedAffiliatesAtQuotedMarketPrice
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | wtm_InvestmentsInUnconsolidatedAffiliatesRollForward
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Loss resulting from the dilutive effect of dividend payments and issuance of restricted shares by an equity method investee. + References No definition available. + Details
Name: | wtm_LossFromDilutiveEffectResultingFromDividendAndIssuanceOfRestrictedShares
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents other comprehensive income increase relating to amortizartion of common share basis difference + References No definition available. + Details
Name: | wtm_Othercomprehensiveincomeincreaserelatingtoamortizartionofcommonsharebasisdifference
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_ExtraordinaryItemAxis=wtm_AmortizationofcommonsharebasisdifferenceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ExtraordinaryItemAxis=wtm_ImpairmentofequityinearningsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ExtraordinaryItemAxis=wtm_ImpairmentofnetunrealizedgainslossesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis=wtm_SymetraMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_WarrantMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=wtm_EquityinearningsofunconsolidatedaffiliatesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |