Description of the period of time over 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, which may be expressed in a variety of ways (for example, in years, month and year).

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Period Type:	duration

- 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

-URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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Namespace Prefix:	us-gaap
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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Namespace Prefix:	us-gaap
Data Type:	xbrli:sharesItemType
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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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ Details

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

- 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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Namespace Prefix:	us-gaap
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Balance Type:	na
Period Type:	duration

- Details

Name:	wtm_AccruedExpenseRollForward
Namespace Prefix:	wtm
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ Details

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Namespace Prefix:	wtm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ Details

Name:	wtm_IncentiveCompensationPayable
Namespace Prefix:	wtm
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents percentage of target achieved by employee under performance shares during the reporting period.

+ Details

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

- Definition

This element represents the target percentage vesting for the performance shares.

+ Details

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

- Definition

This element represents additional compensation cost that would be recognized if all outstanding performance shares are vested.

+ Details

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Namespace Prefix:	wtm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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.

+ Details

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Namespace Prefix:	wtm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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.

+ Details

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

Employee Share-Based Incentive Compensation Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance shares	Sep. 30, 2010 WTM Incentive Plan and WTM Phantom Share Plans Performance shares	Sep. 30, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance shares	Sep. 30, 2010 WTM Incentive Plan and WTM Phantom Share Plans Performance shares	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares	Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Performance shares	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares	Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Performance shares	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2007-2009	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2008-2010	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options	Sep. 30, 2010 Non-Qualified Options	Sep. 30, 2011 Non-Qualified Options	Sep. 30, 2010 Non-Qualified Options
Share-based compensation arrangement by share-based payment award
Vesting period (in years)					3Y										Third, fourth and fifth anniversaries
Target Performance Share activity
Beginning of period (in shares)			150,064	163,220	163,184	172,454	671,727	1,770,423	1,464,295	2,224,215
Shares expired (in shares)					(51,131)	(49,127)		(199,774)	(936,150)	(882,118)
New grants (in shares)					37,675	47,880		14,618	194,900	293,478
Cancellations and change in assumed forfeitures (in shares)				(36)	336	(8,023)	(19,583)	(20,438)	(70,901)	(70,746)
Ending of period (in shares)			150,064	163,184	150,064	163,184	652,144	1,564,829	652,144	1,564,829
Expense Amortized
Accrued expense at the beginning of period	$ 169.9	$ 157.4	$ 56.5	$ 20.2	$ 29.4	$ 12.2	$ 10.0	$ 15.2	$ 18.5	$ 15.1			$ 4.8	$ 4.1	$ 4.5	$ 3.6
Payments and deferrals								(2.2)	(10.5)	(4.5)
Assumed forfeitures and cancellations					(0.6)		0.1	(0.3)	(0.2)	(0.7)
Expense recognized			(1.0)	3.0	26.7	11.0	(0.5)	1.7	1.8	4.5			0.1	0.2	0.4	0.7	10.1	0.1	11.5
Accrued expense at the end of the period	169.9	157.4	55.5	23.2	55.5	23.2	9.6	14.4	9.6	14.4			4.9	4.3	4.9	4.3
Percentage of target achieved by former employees as a result of the Commercial Lines and Personal Lines Transactions (as a percent)									100.00%		14.20%	68.50%
Target vesting percentage of performance shares (as a percent)			100.00%		100.00%		100.00%		100.00%
Additional compensation cost that would be recognized if all outstanding performance shares vested			$ 23.5		$ 23.5		$ 3.7		$ 3.7