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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) (USD $) | 12 Months Ended |  | 12 Months Ended |
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Dec. 31, 2013 One Beacon Employee Stock Ownership Plan | Dec. 31, 2013 Performance Shares [Member] White Mountains multiplier | Dec. 31, 2013 Performance Shares [Member] W T M Incentive and Phantom Plan [Member] | Dec. 31, 2012 Performance Cycle From 2010 to 2012 W T M Incentive and Phantom Plan [Member] | Dec. 31, 2013 W T M Incentive and Phantom Plan [Member] Performance Shares [Member] | Dec. 31, 2013 Performance Cycle From 2010 to 2012 Performance Shares [Member] W T M Incentive and Phantom Plan [Member] | Dec. 31, 2012 Performance Cycle From 2009 to 2011 Performance Shares [Member] | Dec. 31, 2012 Performance Cycle From 2009 to 2011 Performance Shares [Member] W T M Incentive and Phantom Plan [Member] | Dec. 31, 2013 IRS OneBeacon
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 | $ 113,700 |  |  |  |  |  |  |  |
Variable contribution provided to participants (as percentage) | 6.00% |  |  |  |  |  |  |  |
Deferred Benefit Plan Compensation Limits |  |  |  |  |  |  |  |  | 255,000
Multiplier for determining payout, low end of range |  | 0 |  |  |  |  |  |  |
Multiplier for determining payout, high end of range |  | 2 |  |  |  |  |  |  |
Performance cycle period |  | 3 years |  |  |  |  |  |  |
Performance goal percentage for minimum payout |  |  |  |  |  | 33.00% |  | 147.00% |
Payout for maximum growth target percentage |  |  |  |  |  | 98.00% |  | 155.00% |
Partial Payout for 2010 - 2012 cycle |  |  |  | 9.9 |  |  |  |  |
Vested (in shares) |  |  |  |  |  |  | 9,577 |  |
Unamortized grant date fair value |  |  | $ 33,800,000 |  |  |  |  |  |

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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + 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 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 + Details
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- Details
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- Definition Defined Benefit Plan Compensation LImits + 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 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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- 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 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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- Definition Represents the lpercentage of target achieved, high end of range. + References No definition available. + Details
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