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.

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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).

+ References

+ Details

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardCompensationCost
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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
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

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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the noncontrolling interest expense recognized for the dilution of ownership, which resulted from the issuance of the restricted shares.

+ Details

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

- 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

Name:	wtm_PercentageOfGrowthInBookValuePerShareConditionForVesting
Namespace Prefix:	wtm
Data Type:	num:percentItemType
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

Name:	wtm_PercentageOfOtherThanOptionVestingTrancheInEachYear
Namespace Prefix:	wtm
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ Details

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

- Definition

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

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestNumerator
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

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsReductionInGrantTotal
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

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

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	1 Months Ended	3 Months Ended		6 Months Ended						3 Months Ended	6 Months Ended
	May 31, 2011 OneBeacon Restricted Stock Chairman and CEO	May 25, 2011 OneBeacon Restricted Stock Chairman and CEO	Jun. 30, 2011 OneBeacon Restricted Stock	May 31, 2011 Performance shares	Jun. 30, 2011 Performance shares	Jun. 30, 2010 Performance shares	Jun. 30, 2011 Performance shares	Jun. 30, 2010 Performance shares	Mar. 31, 2011 Performance shares	Dec. 31, 2010 Performance shares	Mar. 31, 2010 Performance shares	Dec. 31, 2009 Performance shares	Mar. 31, 2008 Restricted Stock Units, RSU	Jun. 30, 2011 Restricted Stock Units, RSU	Jun. 30, 2010 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)				35,000	46,445	48,363	51,318	50,308
Total number of performance share grants to be reduced (in shares)	210,000
Numerator value of the ratio in which restricted shares vest	1
Denominator value of the ratio in which restricted shares vest	4
Period in which performance share awards will be reduced by a similar number of shares (in years)	5
Expense recognized					$ 1.9	$ (0.6)	$ 2.3	$ 2.8						$ 0.1	$ 0.2
Expense not yet recognized			8.5
Noncontrolling interest expense recognized for the dilution of ownership			$ 6.5
New grants (in shares)						6,449	194,900	278,860					116,270
Proportion of award vests on each of Nov 9, 2009, 2010 and 2011													0.33
Minimum percentage of growth in book value per share for vesting each tranche of units (as a percent)													4.00%
Outstanding (in shares)					671,727	1,770,423	671,727	1,770,423	718,172	1,464,295	1,812,337	2,224,215		23,057
Vesting period if Chairman and CEO was terminated other than for cause, was demoted or suffered a material reduction in compensation (in months)	within 24 months