Employee Share-Based Incentive Compensation Plans (Details) (USD $)	12 Months Ended
Dec. 31, 2014
Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
One Beacon Employee Stock Ownership Plan
Share-based compensation arrangement by share-based payment award
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	117,000
Variable contribution provided to participants (as percentage)	6.00%
OneBeacon \| IRS
Share-based compensation arrangement by share-based payment award
Defined Benefit Plan Compensation LImits	260,000

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.

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

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- Definition

Defined Benefit Plan Compensation LImits

+ References

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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

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- Definition

Represent the amount of contributed by the employee maximum amount applicable to social security wage base.

+ References

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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.

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