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/presentationRef

-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=6415400&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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- Definition

This element represents the number of non-vested performance shares before forfeiture assumptions.

+ References

No definition available.

+ Details

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- Details

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- Details

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Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 121.0		$ 108.1
OneBeacon \| Performance Share \| One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	473,997
Incentive Compensation Payable before Forfeitures	$ 2.1
Target Performance Shares Outstanding, Assumed forfeitures	(11,850)
Accrued Expense, Assumed forfeitures	0.0
Total Target Performance Shares Outstanding	462,147	462,147	517,470	512,938	512,938	493,421
Accrued incentive compensation	$ 2.1	$ 2.4	$ 3.4	$ 5.5	$ 4.4	$ 4.0
OneBeacon \| Performance Share \| One Beacon Long Term Incentive Plan \| Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	154,887
Incentive Compensation Payable before Forfeitures	$ 0.5
OneBeacon \| Performance Share \| One Beacon Long Term Incentive Plan \| Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	151,810
Incentive Compensation Payable before Forfeitures	$ 0.3
OneBeacon \| Performance Share \| One Beacon Long Term Incentive Plan \| Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	167,300
Incentive Compensation Payable before Forfeitures	$ 1.3