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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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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- 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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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: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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- Definition

Represents the percentage 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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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 - Performance Shares Granted (Details) - Performance Shares - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	43,105
Incentive compensation payable before forfeitures	$ 57.3
Assumed forfeitures (in shares)	(647)
Assumed forfeitures	$ (1.0)
Shares outstanding (in shares)	42,458	42,473	40,616	50,515
Accrued incentive compensation	$ 56.3	$ 43.7	$ 31.7	$ 45.8
Performance Cycle 2020 to 2022
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	14,055
Incentive compensation payable before forfeitures	$ 9.5
Targeted performance goal (as a percent)	7.00%
Payout for maximum growth target percentage	200.00%
Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	15,600
Incentive compensation payable before forfeitures	$ 20.7
Targeted performance goal (as a percent)	7.00%
Payout for maximum growth target percentage	200.00%
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Number of nonvested options before forfeitures (in shares)	13,450
Incentive compensation payable before forfeitures	$ 27.1
Targeted performance goal (as a percent)	6.00%
Payout for maximum growth target percentage	200.00%
Minimum \| Performance Cycle 2020 to 2022
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	2.00%
Minimum \| Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	2.00%
Minimum \| Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	2.00%
Maximum \| Performance Cycle 2020 to 2022
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	12.00%
Maximum \| Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	12.00%
Maximum \| Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	10.00%