The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

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.

+ 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 (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=116856206&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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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: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

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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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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

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Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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Employee Share-Based Incentive Compensation Plans (Performance Shares Granted Under the WTM Incentive Plan) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Performance Share \| 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 Share \| 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 \| Restricted Share Cliff Vesting in January 2022 [Member]
Share-based compensation arrangement by share-based payment award
New grants (in shares)		1,900		15,600
White Mountains \| Performance Share
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	43,105		43,105
Accrued Expense	$ 16.0		$ 16.0
Target Performance Shares Outstanding, Assumed forfeitures	(647)		(647)
Accrued Expense, Assumed forfeitures	(0.2)		(0.2)
Total Target Performance Shares Outstanding	42,458	42,473	42,458	42,473	42,458	42,473	40,602	40,616
Total Accrued Expense	$ 15.8	$ 30.8	$ 15.8	$ 30.8	$ 9.8	$ 43.7	$ 23.1	$ 31.7
New grants (in shares)	0	1,900	14,055	15,600
White Mountains \| Performance Share \| Performance Cycle 2018 to 2020
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	13,450		13,450
Accrued Expense	$ 8.0		$ 8.0
White Mountains \| Performance Share \| Performance Cycle 2019 to 2021
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	15,600		15,600
Accrued Expense	$ 5.9		$ 5.9
White Mountains \| Performance Share \| Performance Cycle 2020 to 2022
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	14,055		14,055
Accrued Expense	$ 2.1		$ 2.1