Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified				12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 W T M Incentive Plan Performance Share	Dec. 31, 2012 W T M Incentive Plan Performance Share Performance Cycle From 2010 to 2012	Dec. 31, 2012 W T M Incentive Plan Performance Share Performance Cycle From 2011 to 2013	Dec. 31, 2012 W T M Incentive Plan Performance Share Performance Cycle From 2012 to 2014
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)			117,427	42,320	37,130	37,977
Incentive compensation payable before forfeitures			$ 27.6	$ 7.3	$ 12.7	$ 7.6
Assumed rorfeitures in period (in shares)			(2,935)
Assumed forfeitures in period amount			(0.8)
Shares outstanding (in shares)			114,492
Accrued incentive compensation	$ 159.0	$ 187.9	$ 26.8
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share				11.00%	10.00%	8.00%
Performance goal percentage for minimum payout				4.00%	3.00%	2.00%
Performance goal percentage for maximum payout				18.00%	17.00%	14.00%
Payout for maximum growth target percentage				200.00%	200.00%	200.00%

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 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

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

+ Details

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

- Details

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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_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeitures
Namespace Prefix:	wtm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAssumedForfeituresAmount
Namespace Prefix:	wtm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage growth in intrinsic business value per share to achieve a maximum payment.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage growth in intrinsic business value per share to achieve a minimum payment.

+ References

No definition available.

+ Details

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

- Definition

Represents the targeted percentage of growth in intrinsic business value per share to achieve 100% payment of outstanding performance shares.

+ References

No definition available.

+ Details

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