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://fasb.org/us-gaap/role/ref/legacyRef
-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=116856206&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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
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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

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
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_PlanNameAxis=wtm_WTMIncentivePlanMember
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- Details

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- Details

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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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Range Of Performance Cycle 2019 to 2021
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	7.00%
Payout for maximum growth target percentage	200.00%
Range Of Performance Cycle 2018 to 2020
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	6.00%
Payout for maximum growth target percentage	200.00%
Range Of Performance Cycle From 2017 to 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%
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)	43,120
Incentive compensation payable before forfeitures	$ 44.3
Assumed forfeitures (shares)	(647)
Assumed forfeitures in period amount	(0.6)
Shares outstanding (in shares)	42,473	40,616	50,515	80,353
Accrued incentive compensation	$ 43.7	$ 31.7	$ 45.8	$ 42.4
White Mountains \| Range Of Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	15,600
Incentive compensation payable before forfeitures	$ 6.0
White Mountains \| Range Of Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	13,450
Incentive compensation payable before forfeitures	$ 10.5
White Mountains \| Range Of Performance Cycle From 2017 to 2019
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	14,070
Incentive compensation payable before forfeitures	$ 27.8
Minimum \| Range Of Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum \| Range Of Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum \| Range Of Performance Cycle From 2017 to 2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	1.00%
Maximum \| Range Of Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	12.00%
Maximum \| Range Of Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	10.00%
Maximum \| Range Of Performance Cycle From 2017 to 2019
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	9.00%