Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restricted Share Cliff Vesting in January 2016 [Member]
Target Performance Share activity
Issued (in shares)			25,720
W T M Incentive and Phantom Plan [Member] \| Restricted Share Cliff Vesting in January 2017 [Member]
Target Performance Share activity
Issued (in shares)		23,440
W T M Incentive and Phantom Plan [Member] \| Restricted Share Cliff Vesting in January 2018 [Member]
Target Performance Share activity
Issued (in shares)	23,640
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	83,314	94,130	69,910
Issued (in shares)	23,640	23,440	25,720
Vested (in shares)	(36,279)	(33,205)	(1,500)
Assumed forfeitures and cancellations (in shares)	0	(1,051)	0
End of period (4)	70,675	83,314	94,130
Unamortized grant date fair value
Beginning balance	$ 14.3	$ 17.0	$ 16.8
Issued	15.7	13.1	14.4
Assumed forfeitures and cancellations	0.0	(0.5)	0.0
Expense recognized	(14.3)	(15.3)	(14.2)
Ending balance	$ 15.7	$ 14.3	$ 17.0
White Mountains \| W T M Incentive Plan \| Performance Shares
Target Performance Share activity
Beginning of period	113,198	108,605	108,065
Assumed forfeitures and cancellations (in shares)	(3,857)	(3,257)	(20)
End of period (4)	96,211	113,198	108,605
Unamortized grant date fair value
Assumed forfeitures and cancellations	$ (0.3)	$ (0.1)	$ (0.6)
Expense recognized	(44.4)	$ (14.1)	$ (39.1)
Ending balance	$ 22.5
Range of Performance Cycle from 2011 to 2013 [Member] \| Performance Shares
Target Performance Share activity
Vested (in shares)		(3,570)
Range of Performance Cycle from 2011 to 2013 [Member] \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Percentage of payout for minimum growth target percentage		37.10%
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive and Phantom Plan [Member] \| Restricted Stock
Target Performance Share activity
Assumed forfeitures and cancellations (in shares)		(3,040)
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive and Phantom Plan [Member] \| Performance Shares
Target Performance Share activity
Assumed forfeitures and cancellations (in shares)	(3,040)

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 14
-Section F

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

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)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
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://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

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

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)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
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 share instruments newly issued under a share-based compensation plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The total reduction to unamortized compensation cost for performance shares forfeited during the period.

+ References

No definition available.

+ Details

Name:	wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsForfeitedInPeriodTotalCompensationCostNotYetRecognized
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total fair value (on the date of issue) for restricted share awards issued during the period.

+ References

No definition available.

+ Details

Name:	wtm_EmployeeServiceShareBasedCompensationNonvestedAwardsIssuedInPeriodTotalCompensationCostNotYetRecognized
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	wtm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantDateFairValueRollForward
Namespace Prefix:	wtm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the lpercentage 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=wtm_RestrictedShareCliffVestingInJanuary2016Member
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_RestrictedShareCliffVestinginJanuary2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=wtm_RestrictedShareCliffVestinginJanuary2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=wtm_WhiteMountainsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_RangeOfPerformanceCycleFrom2011To2013Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	wtm_ShareBasedCompensationArrangementByPerformanceSharesAwardPerformanceCycleAxis=wtm_RangeOfPerformanceCycleFrom20142016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: