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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) (USD $) |  |  | 12 Months Ended | 9 Months Ended | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2012 Performance Share multiplier | Dec. 31, 2012 W T M Incentive and Phantom Plan Performance Share | Dec. 31, 2011 W T M Incentive and Phantom Plan Performance Share | Dec. 31, 2010 W T M Incentive and Phantom Plan Performance Share | Dec. 31, 2012 W T M Incentive and Phantom Plan Performance Cycle From 2010 to 2012 | Dec. 31, 2012 One Beacon Employee Stock Ownership Plan | Dec. 31, 2012 Sirius Groups Profit Sharing Plan | Dec. 31, 2012 W T M Incentive and Phantom Plan Performance Share | Sep. 30, 2012 Performance Cycle From 2009 to 2011 Performance Share | Dec. 31, 2012 Performance Cycle From 2009 to 2011 W T M Incentive and Phantom Plan Performance Share | Dec. 31, 2012 One Beacon Segment
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period (in years) |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |  |
Annual base contribution provided to participants expressed as percentage of salary |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |
Social security wage base amount maximum |  |  |  |  |  |  |  |  |  |  | $ 110,100 |  |  |  |  |
Variable contribution provided to participants not eligible to certain benefits expressed as percentage of salary |  |  |  |  |  |  |  |  |  |  | 6.00% |  |  |  |  |
Employee stock ownership Plan (ESOP), salary amount maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 250,000
Variable contribution provided to participants expressed as percentage of salary |  |  |  |  |  |  |  |  |  |  |  | 7.00% |  |  |  |
Multiplier for determining payout, low end of range |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |
Multiplier for determining payout, high end of range |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |
Performance cycle period |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 150,064 |  | 163,184 |  | 172,454 |  |  |  |  |  |  |  |
Shares paid or expired (in shares) |  |  |  | (68,357) | [1] | (51,131) | [1] | (49,127) | [1] |  |  |  |  |  |  |
New grants (in shares) |  |  |  | 38,432 |  | 37,675 |  | 47,880 |  |  |  |  |  |  |  |
Assumed forfeitures and cancellations (in shares) |  |  |  | (782) | [2] | 336 | [2] | (8,023) | [2] |  |  |  |  |  |  |
Ending of period (in shares) |  |  |  | 119,357 |  | 150,064 |  | 163,184 |  |  |  |  |  |  |  |
Accrued Expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued expense at the beginning of period | 159,000,000 | 187,900,000 |  | 66,100,000 |  | 29,400,000 |  | 12,200,000 |  |  |  |  |  |  |  |
Payments and deferrals |  |  |  | (58,300,000) | [1] | 0 | [1] | 0 | [1] |  |  |  |  |  |  |
Assumed forfeitures and cancelations |  |  |  | 600,000 | [2] | (900,000) | [2] | (700,000) | [2] |  |  |  |  |  |  |
Expense recognized |  |  |  | 21,000,000 |  | 37,600,000 |  | 17,900,000 |  |  |  |  |  |  |  |
Accrued expense at the end of the period | 159,000,000 | 187,900,000 |  | 29,400,000 |  | 66,100,000 |  | 29,400,000 |  |  |  |  |  |  |  |
Performance goal percentage for minimum payout |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 147.00% |
Payout for maximum growth target percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 155.00% |
Partial Payout for 2010 - 2012 cycle |  |  |  |  |  |  |  |  |  | 9.9 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period |  |  |  |  |  |  |  |  |  |  |  |  |  | 9,577 |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested |  |  |  | $ 21,100,000 |  |  |  |  |  |  |  |  |  |  |  |
[1] | WTM performance share payments in 2012 for the 2009-2011 performance cycle ranged from 147% to 155% of target. At December 31, 2012, White Mountains paid $9.9 as a partial payment for the 2010-2012 performance cycle. There were no payments made in 2011 and 2010 for the 2008-2010 and 2007-2009 performance cycles; those performance shares did not meet the threshold performance goals and expired.
[2] | Amounts include changes in assumed forfeitures, as required under GAAP.

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph g(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(e) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(c) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(b) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(a) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(d) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition The total reduction to unamortized compensation cost for performance shares forfeited during the period. + References No definition available. + Details
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- Definition Represents the employer's annual contribution (as a percentage of salary up to the Social Security wage base) to the ESOP plan. + References No definition available. + Details
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- Definition Employee Stock Ownership Plan (ESOP), Salary Amount Maximum + References No definition available. + Details
Name: | wtm_EmployeeStockOwnershipPlanEsopSalaryAmountMaximum
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represent the amount of contributed by the employee maximum amount applicable to social security wage base. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the employer's maximum annual variable contribution (as a percentage of salary up to the Social Security wage base) to the ESOP plan for employees who are not otherwise eligible for certain incentive compensation benefits. + References No definition available. + Details
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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 Partial Payout for 2010 - 2012 cycle + References No definition available. + Details
Name: | wtm_PartialPayoutFor20102012Cycle
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Balance Type: | debit
Period Type: | duration

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- Definition Represents the employer's maximum annual variable contribution (as a percentage of salary) to the profit sharing plan for employees. + References No definition available. + Details
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Namespace Prefix: | wtm_
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Balance Type: | na
Period Type: | duration

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- Definition This element represents additional compensation cost that would be recognized if all outstanding performance shares are vested. + References No definition available. + Details
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- Definition The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + References No definition available. + Details
Name: | wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTargetSharesPaidInPeriod
Namespace Prefix: | wtm_
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Balance Type: | na
Period Type: | duration

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- Definition Represents the high end of the range of multiplier for number of shares initially granted to determine payout. + References No definition available. + Details
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- Definition Represents the low end of the range of the multiplier for the number of shares initially granted to determine payout. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Represents the lpercentage of target achieved, high end of range. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Represents the performance cycle on conclusion of which shares become payable. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Represents the percentage growth in intrinsic business value per share to achieve a minimum payment. + References No definition available. + Details
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