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Employee Share-Based Incentive Compensation Plans (WTM Phantom Performance Shares Granted Under WTM Phantom Shares Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 159.0		$ 187.9
Performance Share | W T M Phantom Share Plans
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	4,990
Incentive compensation payable before forfeitures	2.6
Assumed rorfeitures in period (in shares)	(125)
Assumed forfeitures in period amount	0
Shares outstanding (in shares)	4,865
Accrued incentive compensation	2.6
Performance Share | Performance Cycle From 2010 to 2012 | W T M Phantom Share Plans
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	4,990
Incentive compensation payable before forfeitures	2.6
Performance Share | Performance Cycle From 2011 to 2013 | W T M Phantom Share Plans
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	0	[1]
Incentive compensation payable before forfeitures	0	[1]
Performance Share | Performance Cycle From 2012 to 2014 | W T M Phantom Share Plans
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	0	[1]
Incentive compensation payable before forfeitures	0	[1]
W T M Incentive Plan | Performance Share
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	117,427
Incentive compensation payable before forfeitures	27.6
Assumed rorfeitures in period (in shares)	(2,935)
Assumed forfeitures in period amount	0.8
Shares outstanding (in shares)	114,492
Accrued incentive compensation	26.8
W T M Incentive Plan | Performance Cycle From 2010 to 2012 | Performance Share
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	42,320
Incentive compensation payable before forfeitures	7.3
W T M Incentive Plan | Performance Cycle From 2011 to 2013 | Performance Share
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	37,130
Incentive compensation payable before forfeitures	12.7
W T M Incentive Plan | Performance Cycle From 2012 to 2014 | Performance Share
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	37,977
Incentive compensation payable before forfeitures	$ 7.6
[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 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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
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 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

X

- 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

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