Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Expense Amortized
Accrued expense at the beginning of period			$ 184.6wtm_IncentiveCompensationPayable
Accrued expense at the end of the period	100.5wtm_IncentiveCompensationPayable		184.6wtm_IncentiveCompensationPayable
OneBeacon \| Performance Share \| One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period	3 years
OneBeacon \| One Beacon Long Term Incentive Plan \| Range Of Performance Cycle From 2012 To 2014 [Member]
Expense Amortized
Percentage of target payout - minimum actual	45.70%
OneBeacon \| One Beacon Long Term Incentive Plan \| Performance Cycle 2011 to 2013
Expense Amortized
Percentage of target payout - minimum actual	37.10%
OneBeacon \| One Beacon Long Term Incentive Plan \| Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	517,470	493,421
Shares paid or expired (in shares)	(181,290)	(142,138)
New grants (in shares)	154,887	165,800
Assumed forfeitures and cancellations	(28,920)	(4,145)
Ending of period (in shares)	462,147	512,938
Expense Amortized
Accrued expense at the beginning of period	3.4	4.0
Payments and deferrals	(1.5)	(1.0)
Assumed forfeitures and cancellations	0	0
Expense recognized	0.2	0.6
Accrued expense at the end of the period	2.1	3.6
Additional compensation cost that would be recognized if all outstanding performance shares vested	3.3

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 50

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

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Unrecognized cost of unvested share-based compensation awards.

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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.

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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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).

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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.

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- Definition

The total reduction to unamortized compensation cost for performance shares forfeited during the period.

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- Definition

Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements.

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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.

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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.

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No definition available.

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- Definition

Represents the lpercentage of target achieved, low end of range.

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