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Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012
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Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Total Accrued Expense | $ 157.9 |  | $ 218.3 |  |  |
White Mountains | Performance Share | W T M Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 130,807 |  |  |  |  |
Accrued Expense | 51.8 |  |  |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (3,270) |  |  |  |  |
Accrued Expense, Assumed forfeitures | (1.3) |  |  |  |  |
Total Target Performance Shares Outstanding | 127,537 | 127,537 | 119,220 | 118,976 | 118,976 | 119,357
Total Accrued Expense | 50.5 | 42.3 | 60.2 | 34.9 | 28.6 | 29.4
White Mountains | Performance Cycle 2012 To 2014 | Performance Share | W T M Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 37,977 |  |  |  |  |
Accrued Expense | 28.0 |  |  |  |  |
White Mountains | Performance Cycle 2014 to 2016 | Performance Share | W T M Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 45,660 |  |  |  |  |
Accrued Expense | 4.6 |  |  |  |  |
White Mountains | Performance Cycle 2013 to 2015 | Performance Share | W T M Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 47,170 |  |  |  |  |
Accrued Expense | $ 19.2 |  |  |  |  |

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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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

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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
Name: | wtm_IncentiveCompensationPayable
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
Name: | wtm_IncentiveCompensationPayableBeforeForfeitures
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeitures
Namespace Prefix: | wtm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeituresAmount
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures
Namespace Prefix: | wtm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant