Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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 (i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

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 (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

Number of shares issued under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the high end of the range of multiplier for number of shares initially granted to determine payout.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardMultiplierForDeterminingPayoutHighEndOfRange
Namespace Prefix:	wtm_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the low end of the range of the multiplier for the number of shares initially granted to determine payout.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardMultiplierForDeterminingPayoutLowEndOfRange
Namespace Prefix:	wtm_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of target achieved, high end of range.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMaximumGrowthTargetPercentage
Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the performance cycle on conclusion of which shares become payable.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceCyclePeriod
Namespace Prefix:	wtm_
Data Type:	xbrli:durationItemType
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:	dtr-types: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:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_PerformanceCycleRange1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_PerformanceCycleRange3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_PerformanceCycleRange2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=wtm_RangeOfPerformanceCycleFrom2021To2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=wtm_WTMIncentiveAndPhantomPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=wtm_RestrictedShareCliffVestingInJanuary2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=wtm_RestrictedShareCliffVestinginJanuary2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=wtm_RestrictedShareCliffVestingInJanuary2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Employee Share-Based Incentive Compensation Plans - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) multiplier shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)
Performance Shares
Share-based compensation arrangement by share-based payment award
Vesting period	3 years
Multiplier for determining payout, low end of range \| multiplier	0
Multiplier for determining payout, high end of range \| multiplier	2
Performance cycle period	3 years
Unamortized grant date fair value \| $	$ 12.0
Performance Shares \| 2021 – 2023
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	8.00%
Payout for maximum growth target percentage	200.00%
Performance Shares \| 2019 – 2021
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	7.00%
Payout for maximum growth target percentage	200.00%
Performance Shares \| 2020 – 2022
Share-based compensation arrangement by share-based payment award
Targeted performance goal (as a percent)	7.00%
Payout for maximum growth target percentage	200.00%
Performance Shares \| Minimum \| 2021 – 2023
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	3.00%
Performance Shares \| Minimum \| 2019 – 2021
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	2.00%
Performance Shares \| Minimum \| 2020 – 2022
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	2.00%
Performance Shares \| Maximum \| 2021 – 2023
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	13.00%
Performance Shares \| Maximum \| 2019 – 2021
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	12.00%
Performance Shares \| Maximum \| 2020 – 2022
Share-based compensation arrangement by share-based payment award
Payout for minimum growth target percentage	12.00%
Range Of Performance Cycle From 2021 to 2023 \| W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)	13,475
Restricted Share Cliff Vesting in January 2023 \| W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)		14,055
Restricted Share Cliff Vesting in January 2022 \| W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)			15,600
Restricted Stock
Share-based compensation arrangement by share-based payment award
Vesting period	3 years
Performance shares (in shares)	13,475	14,055	15,600
Unamortized grant date fair value \| $	$ 15.9	$ 15.2	$ 16.7	$ 12.5
Restricted Share Cliff Vesting in January 2024 \| W T M Incentive and Phantom Plan
Share-based compensation arrangement by share-based payment award
Performance shares (in shares)	13,475