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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- 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.

+ References

No definition available.

+ 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.

+ References

No definition available.

+ 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.

+ References

No definition available.

+ 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.

+ References

No definition available.

+ 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.

+ References

No definition available.

+ 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, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 90.0	$ 187.9
WTM Incentive Plan \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	116,497
Accrued Expense	26.0
Assumed forfeitures (in shares)	(2,912)
Assumed forfeitures	(0.7)
Target Performance Shares Outstanding	113,585
Accrued incentive compensation	25.3
WTM Incentive Plan \| Performance cycle 2010-2012 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	42,890
Accrued Expense	15.4
WTM Incentive Plan \| Performance cycle 2011-2013 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	37,675
Accrued Expense	8.9
WTM Incentive Plan \| Performance cycle 2012-2014 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	35,932
Accrued Expense	1.7
WTM Phantom Share Plans \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	4,990
Accrued Expense	2.1
Assumed forfeitures (in shares)	(125)
Target Performance Shares Outstanding	4,865
Accrued incentive compensation	2.1
WTM Phantom Share Plans \| Performance cycle 2010-2012 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	4,990
Accrued Expense	2.1
OneBeacon Long-Term Incentive Plan \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	584,119
Accrued Expense	3.3
Assumed forfeitures (in shares)	(14,374)
Assumed forfeitures	(0.1)
Target Performance Shares Outstanding	569,745	642,667	718,172	1,464,295
Accrued incentive compensation	3.2	9.7	8.4	18.5
OneBeacon Long-Term Incentive Plan \| Performance cycle 2009-2011 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	9,168
Accrued Expense	0.3
OneBeacon Long-Term Incentive Plan \| Performance cycle 2010-2012 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	242,098
Accrued Expense	1.6
OneBeacon Long-Term Incentive Plan \| Performance cycle 2011-2013 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	151,563
Accrued Expense	1.1
OneBeacon Long-Term Incentive Plan \| Performance cycle 2012-2014 \| Performance shares
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	181,290
Accrued Expense	$ 0.3