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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
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: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 of target achieved, low end of range.

+ References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardPayoutForMinimumGrowthTargetPercentage
Namespace Prefix:	wtm_
Data Type:	num: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:	us-gaap_PlanNameAxis=wtm_WTMIncentivePlanMember
Namespace Prefix:
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- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_RangeOfPerformanceCycleFrom20172019Member
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- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_RangeOfPerformanceCycleFrom20162018Member
Namespace Prefix:
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- Details

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Namespace Prefix:
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- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_PerformanceCycle2018to2020Member
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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	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Percentage of target payout - minimum actual	174.00%
Percentage of target payout - maximum actual	180.00%
Performance Cycle 2016 to 2018
Share-based compensation arrangement by share-based payment award
Percentage of target payout - minimum actual		139.00%
Percentage of target payout - maximum actual		166.00%
White Mountains
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	43,105
Accrued Expense	$ 10.0
Target Performance Shares Outstanding, Assumed forfeitures	(647)
Accrued Expense, Assumed forfeitures	(0.2)
Total Target Performance Shares Outstanding	42,458	40,602	42,473	40,616
Total Accrued Expense	$ 9.8	$ 23.1	$ 43.7	$ 31.7
White Mountains \| Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450
Accrued Expense	$ 5.1
White Mountains \| Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	15,600
Accrued Expense	$ 3.8
White Mountains \| Performance Cycle 2020 to 2022
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,055
Accrued Expense	$ 1.1