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Employee Share-Based Incentive Compensation Plans (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 113.6			$ 187.9
Range of Performance Cycle from 2010 to 2012 [Member] | W T M Phantom Share Plans [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	4,990
Incentive Compensation Payable before Forfeitures	2.4
Range of Performance Cycle from 2011 to 2013 [Member] | W T M Phantom Share Plans [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	0	[1]
Incentive Compensation Payable before Forfeitures	0	[1]
Range of Performance Cycle from 2012 to 2014 [Member] | W T M Phantom Share Plans [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	0	[1]
Incentive Compensation Payable before Forfeitures	0	[1]
Performance Share [Member] | W T M Phantom Share Plans [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	4,990
Incentive Compensation Payable before Forfeitures	2.4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period	(125)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period Amount	0
Target Performance Shares Outstanding	4,865
Accrued incentive compensation	2.4
Performance Share [Member] | OneBeacon Long-Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target vesting percentage of performance shares	100.00%
WTM Incentive Plan | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	116,497
Incentive Compensation Payable before Forfeitures	31.2
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period	(2,912)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period Amount	0.9
Target Performance Shares Outstanding	113,585
Accrued incentive compensation	30.3
WTM Incentive Plan | Range of Performance Cycle from 2010 to 2012 [Member] | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	42,890
Incentive Compensation Payable before Forfeitures	17.2
WTM Incentive Plan | Range of Performance Cycle from 2011 to 2013 [Member] | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	37,675
Incentive Compensation Payable before Forfeitures	10.7
WTM Incentive Plan | Range of Performance Cycle from 2012 to 2014 [Member] | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	35,932
Incentive Compensation Payable before Forfeitures	3.3
OneBeacon Long-Term Incentive Plan | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	574,951
Incentive Compensation Payable before Forfeitures	3.2
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period	(14,374)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period Amount	(0.1)
Target Performance Shares Outstanding	560,577		569,745	642,667	671,727	718,172	1,464,295
Accrued incentive compensation	3.1		3.2	9.7	10.0	8.4	18.5
Additional compensation cost that would be recognized if all outstanding performance shares vested	3.4
OneBeacon Long-Term Incentive Plan | Range of Performance Cycle from 2010 to 2012 [Member] | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	242,098
Incentive Compensation Payable before Forfeitures	1.7
OneBeacon Long-Term Incentive Plan | Performance cycle 2009-2011 | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Percentage of payout for minimum growth target percentage	138.60%
OneBeacon Long-Term Incentive Plan | Range of Performance Cycle from 2011 to 2013 [Member] | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	151,563
Incentive Compensation Payable before Forfeitures	1.1
OneBeacon Long-Term Incentive Plan | Range of Performance Cycle from 2012 to 2014 [Member] | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Nonvested, Number before Forfeitures	181,290
Incentive Compensation Payable before Forfeitures	$ 0.4
OneBeacon Long-Term Incentive Plan | Performance cycle 2008-2010 | Performance Share [Member]
Share-based compensation arrangement by share-based payment award
Percentage of payout for minimum growth target percentage	68.50%
[1] (1) All performance shares for the 2011–2013 and 2012–2014 performance cycles were granted from the WTM Incentive Plan

X

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

X

- Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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

X

- 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

X

- Definition

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

+ References

No definition available.

+ Details

Name:	wtm_PerformanceShareTargetVestingPercentage
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

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

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardAdditionalCostToBeRecognizedIfPerformanceSharesVested
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- 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

X

- 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

X

- 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

X

- Definition

Represents the lpercentage of target achieved, low end of range.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMinimumGrowthTargetPercentage
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration