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

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

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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

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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 growth in intrinsic business value per share to achieve payment.

+ References

No definition available.

+ Details

Name:	wtm_SharebasedCompensationArrangementbySharebasedPaymentAwardPerformanceGoalPercentageforPayout
Namespace Prefix:	wtm_
Data Type:	num: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

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

- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_PlanNameAxis=wtm_WTMIncentivePlanMember
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- Details

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Namespace Prefix:
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- Details

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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 141.7	$ 108.1
Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	9.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2012 to 2014 \| W T M Incentive and Phantom Plan [Member] \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	91.00%
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	145.50%
White Mountains \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	98,678
Incentive compensation payable before forfeitures	$ 59.2
forfeitures	(2,467)
Assumed forfeitures in period amount	(1.5)
Shares outstanding (in shares)	96,211	113,198	108,605	108,065
Accrued incentive compensation	$ 57.7	$ 44.4	$ 54.9	$ 26.5
White Mountains \| Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	29,828
Incentive compensation payable before forfeitures	$ 8.3
White Mountains \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	32,557
Incentive compensation payable before forfeitures	$ 12.9
White Mountains \| Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	36,293
Incentive compensation payable before forfeitures	$ 38.0
Sirius Group \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Shares outstanding (in shares)	7,997	10,351	10,615
Minimum \| Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Minimum \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	3.00%
Minimum \| Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Maximum \| Range Of Performance Cycle From 2015 to 2017 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%
Maximum \| Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	15.00%
Maximum \| Performance Cycle From 2013 to 2015 \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%