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

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

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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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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

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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

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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	xbrli:integerItemType
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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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- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
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Balance Type:	na
Period Type:	duration

- 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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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, 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
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	9.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2012 to 2014 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
White Mountains \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	126,717
Incentive compensation payable before forfeitures	49.4
forfeitures	(3,168)
Assumed forfeitures in period amount	(1.3)
Shares outstanding (in shares)	123,549	119,220	119,357	150,064
Accrued incentive compensation	48.1	60.2	29.4	66.1
White Mountains \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	41,937
Incentive compensation payable before forfeitures	4.1
White Mountains \| Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	46,803
Incentive compensation payable before forfeitures	18.9
White Mountains \| Performance Cycle From 2012 to 2014 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	37,977
Incentive compensation payable before forfeitures	26.4
Minimum \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	3.00%
Minimum \| Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum \| Performance Cycle From 2012 to 2014 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Maximum \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	15.00%
Maximum \| Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%
Maximum \| Performance Cycle From 2012 to 2014 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%