As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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- Definition

Equity-based compensation cost during the period with respect to the award, which will be recognized in income (as well as the total recognized tax benefit) or capitalized as part of the cost of an asset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 50

-Section 50

-Paragraph 1

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(1)

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 64, 65

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 5: 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

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

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Balance Type:	na
Period Type:	duration

- 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 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.

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 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

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

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- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

+ Details

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Period Type:	instant

- Definition

This element represents the minimum percentage of growth in book value per share from January 1, 2008 through the end of the calendar year immediately following the applicable vesting date required to trigger the vesting of restricted stock units.

+ Details

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Namespace Prefix:	wtm
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Balance Type:	na
Period Type:	duration

- Definition

This element represents the percentage of RSUs that vest on each of November 2009, 2010 and 2011.

+ Details

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Namespace Prefix:	wtm
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the denominator value of the ratio in which shares will vest.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Represents the numerator value of the ratio in which shares will vest.

+ Details

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Namespace Prefix:	wtm
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the total number of performance share grants that will be reduced.

+ Details

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Namespace Prefix:	wtm
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of years that the CEO of a subsidiary will forfeit performance shares of that subsidiary.

+ Details

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Employee Share-Based Incentive Compensation Plans (Details 6) (OneBeacon Long-Term Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended	9 Months Ended			3 Months Ended		9 Months Ended						3 Months Ended	9 Months Ended
	May 31, 2011 OneBeacon Restricted Stock Chairman and CEO year	Sep. 30, 2011 OneBeacon Restricted Stock Chairman and CEO	Sep. 30, 2011 OneBeacon Restricted Stock Chairman and CEO	May 25, 2011 OneBeacon Restricted Stock Chairman and CEO	Sep. 30, 2011 OneBeacon Restricted Stock	Sep. 30, 2011 Performance shares	Sep. 30, 2010 Performance shares	Sep. 30, 2011 Performance shares	Sep. 30, 2010 Performance shares	Jun. 30, 2011 Performance shares	Dec. 31, 2010 Performance shares	Jun. 30, 2010 Performance shares	Dec. 31, 2009 Performance shares	Mar. 31, 2008 Restricted Stock Units, RSU	Sep. 30, 2010 Restricted Stock Units, RSU	Sep. 30, 2011 Restricted Stock Units, RSU
Share-based compensation arrangement by share-based payment award
One Beacon Restricted shares issued (in shares)				630,000
Forfeiture of performance shares granted for the 2011 - 2013 performance cycle (in shares)						19,583	20,438	70,901	70,746
Total number of performance share grants to be reduced (in shares)	35,000
Numerator value of the ratio in which restricted shares vest (as a percent)	1
Denominator value of the ratio in which restricted shares vest (as a percent)	4
Period in which performance share awards will be reduced by a similar number of shares (in years)	5
Expense recognized		$ 0.4	$ 0.5			$ (0.5)	$ 1.7	$ 1.8	$ 4.5						$ 0.2
Expense not yet recognized					$ 8.1
New grants (in shares)							14,618	194,900	293,478					116,270
Proportion of award vests on each of Nov 9, 2009, 2010 and 2011 (as a percent)														33.00%
Minimum percentage of growth in book value per share for vesting each tranche of units (as a percent)														4.00%
Outstanding (in shares)		630,000	630,000			652,144	1,564,829	652,144	1,564,829	671,727	1,464,295	1,770,423	2,224,215			22,773