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=120381028&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
Namespace Prefix:	us-gaap_
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:monetaryItemType
Balance Type:	debit
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

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_PlanNameAxis=wtm_WTMIncentivePlanMember
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- Details

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- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_PerformanceCyclePeriodTwoMember
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- Details

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Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	41,405
Assumed forfeitures (in shares)	(621)
Total target performance shares outstanding (in shares)	40,784	42,458	42,458	42,473
Accrued Expense	$ 38.8
Assumed forfeitures	(0.6)
Total Accrued Expense	$ 38.2	$ 56.3	$ 9.8	$ 43.7
2019 – 2021
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	14,850
Accrued Expense	$ 24.9
2020 – 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,555
Accrued Expense	$ 12.7
2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,000
Accrued Expense	$ 1.2