Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Expense Amortized
Ending December 31,	$ 184.6wtm_IncentiveCompensationPayable	$ 218.3wtm_IncentiveCompensationPayable
Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage			138.60%
Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	125,000
Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding Amended, Exercise Price	$ 742
OneBeacon \| Performance Shares \| Long Term Incentive Plan
Target options outstanding
Beginning of period	493,421	563,190	642,667
Shares paid or expired (in shares)	(142,138)	(238,658)	(258,901)
New grants	165,800	179,000	181,290
Assumed forfeitures and cancellations (in shares)	387	(10,111)	(1,866)
Ending December 31,	517,470	493,421	563,190
Expense Amortized
Beginning of period	4.0	1.2	9.7
Payments and deferrals	(1.0)	0	(7.7)
Assumed forfeitures and cancellations	0	(0.1)	0
Expense recognized	0.4	2.9	(0.8)
Ending December 31,	$ 3.4	$ 4.0	$ 1.2
Range of Performance Cycle from 2011 to 2013 [Member] \| Long Term Incentive Plan
Expense Amortized
Percentage of payout for minimum growth target percentage	37.10%

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.

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-Publisher FASB

-Name Accounting Standards Codification

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-Publisher SEC

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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

Number of options outstanding, including both vested and non-vested options.

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- Definition

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

+ References

No definition available.

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- Definition

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

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No definition available.

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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

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- Definition

This element represents the amended exercise price for the non-qualified options.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

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