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=96867065&loc=d3e5070-113901

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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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

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumberBeforeForfeitures
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 payment.

+ References

No definition available.

+ Details

Name:	wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout
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

Name:	wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardTargetedPerformanceGoalforPersonnelExpressedasPercentageofGrowthinIntrinsicBusinessValue
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
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Period Type:	duration

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Namespace Prefix:
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- Details

Name:	us-gaap_PlanNameAxis=wtm_WTMIncentivePlanMember
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- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - W T M Incentive Plan - Performance Shares - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Range Of Performance Cycle From 2017-2019
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	5.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2016-2018
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	4.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2015-2017
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
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	51,285
Incentive compensation payable before forfeitures	$ 46.6
Assumed forfeitures	(770)
Assumed forfeitures in period amount	(0.8)
Shares outstanding (in shares)	50,515	80,353	93,654	111,257
Accrued incentive compensation	$ 45.8	$ 42.4	$ 57.7	$ 44.4
White Mountains \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	16,680
Incentive compensation payable before forfeitures	$ 8.3
White Mountains \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	16,235
Incentive compensation payable before forfeitures	$ 14.7
White Mountains \| Range Of Performance Cycle From 2015-2017
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	18,370
Incentive compensation payable before forfeitures	$ 23.6
Minimum \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	1.00%
Minimum \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	0.00%
Minimum \| Range Of Performance Cycle From 2015-2017
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Maximum \| Range Of Performance Cycle From 2017-2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	9.00%
Maximum \| Range Of Performance Cycle From 2016-2018
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	8.00%
Maximum \| Range Of Performance Cycle From 2015-2017
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%