Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

+ Details

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Balance Type:	na
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- Definition

Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements.

+ References

No definition available.

+ Details

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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

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Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage of performance shares earned

+ References

No definition available.

+ Details

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

- Definition

This element represents the reduction for assumed forfeitures to the number of performance shares outstanding.

+ References

No definition available.

+ Details

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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

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Targeted Performance Goal for Personnel Expressed as Percentage of Growth in Intrinsic Business Value

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	num:percentItemType
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Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 184.6wtm_IncentiveCompensationPayable	$ 218.3wtm_IncentiveCompensationPayable
OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	526,090
Incentive compensation payable before forfeitures	3.4
forfeitures	(8,620)
Assumed forfeitures in period amount	0
Shares outstanding (in shares)	517,470	493,421	563,190	642,667
Accrued incentive compensation	3.4	4.0	1.2	9.7
Unamortized grant date fair value	1.6
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	14.00%	13.00%	10.00%
Range Of Performance Cycle From 2014-2016 [Member] \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	165,800
Incentive compensation payable before forfeitures	0.5
Performance Cycle From 2013 to 2015 \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	179,000
Incentive compensation payable before forfeitures	1.4
Performance Cycle From 2012 to 2014 \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	181,290
Incentive compensation payable before forfeitures	$ 1.5
Maximum \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	21.00%	20.00%	17.00%
Percentage of performance shares earned	200.00%	200.00%	200.00%
Minimum \| OneBeacon \| Performance Shares \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	7.00%	6.00%	3.00%