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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(ii)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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- 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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- 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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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

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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

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Namespace Prefix:	wtm_
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Employee Share-Based Incentive Compensation Plans - Performance Shares Granted (Details) - Performance Shares - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	40,050
Incentive compensation payable before forfeitures	$ 68.5
Assumed forfeitures (in shares)	(601)
Assumed forfeitures	$ (1.0)
Shares outstanding (in shares)	39,449	40,828	42,458	42,473
Accrued incentive compensation	$ 67.5	$ 42.2	$ 56.3	$ 43.7
2022 – 2024
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	13,225
Incentive compensation payable before forfeitures	$ 11.0
2021 – 2023
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	13,475
Incentive compensation payable before forfeitures	$ 19.7
2020 – 2022
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	13,350
Incentive compensation payable before forfeitures	$ 37.8