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.

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

- 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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
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

Carrying value as of the balance sheet date for performance shares before forfeitures.

+ Details

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

- Definition

This element represents the reduction for assumed forfeitures to the number of performance shares outstanding.

+ Details

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

- Definition

This element represents the reduction to accrued expense for assumed forfeitures of performance shares.

+ Details

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

- Definition

This element represents the number of non-vested performance shares before forfeiture assumptions.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures
Namespace Prefix:	wtm
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Employee Share-Based Incentive Compensation Plans (Details 2) (USD $) In Millions, except Share data	6 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 153.3		$ 157.4
WTM Incentive Plan \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	140,975
Accrued Expense	51.5
Assumed forfeitures (in shares)	(3,524)
Assumed forfeitures	(1.3)
Target WTM Performance Shares Outstanding	137,451
Accrued incentive compensation	50.2
WTM Incentive Plan \| Performance shares \| Performance cycle 2009-2011
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	60,410
Accrued Expense	36.6
WTM Incentive Plan \| Performance shares \| Performance cycle 2010-2012
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	42,890
Accrued Expense	11.7
WTM Incentive Plan \| Performance shares \| Performance cycle 2011-2013
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	37,675
Accrued Expense	3.2
WTM Phantom Share Plans \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	12,937
Accrued Expense	6.4
Assumed forfeitures (in shares)	(324)
Assumed forfeitures	(0.1)
Target WTM Performance Shares Outstanding	12,613
Accrued incentive compensation	6.3
WTM Phantom Share Plans \| Performance shares \| Performance cycle 2009-2011
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	7,947
Accrued Expense	4.9
WTM Phantom Share Plans \| Performance shares \| Performance cycle 2010-2012
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	4,990
Accrued Expense	1.5
OneBeacon Long-Term Incentive Plan \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	688,951
Accrued Expense	10.3
Assumed forfeitures (in shares)	(17,224)
Assumed forfeitures	(0.3)
Target WTM Performance Shares Outstanding	671,727	718,172	1,464,295	1,770,423	1,812,337	2,224,215
Accrued incentive compensation	10.0	8.4	18.5	15.2	16.2	15.1
OneBeacon Long-Term Incentive Plan \| Performance shares \| Performance cycle 2009-2011
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	261,016
Accrued Expense	2.4
OneBeacon Long-Term Incentive Plan \| Performance shares \| Performance cycle 2008-2010
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	268,035
Accrued Expense	7.5
OneBeacon Long-Term Incentive Plan \| Performance shares \| Performance cycle 2010-2012
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding (in shares)	159,900
Accrued Expense	$ 0.4