This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities.

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

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 9

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=16385135&loc=d3e33749-111570

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.12)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 7

+ Details

Name:	us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Article 4

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Income Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6515339

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

+ Details

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

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates.

+ References

No definition available.

+ Details

Name:	wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The net unrealized gains (losses) from Symetra's fixed maturity portfolio.

+ References

No definition available.

+ Details

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

- Definition

The value of the exercise of warrants.

+ References

No definition available.

+ Details

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

- Definition

The increase (decrease) in the fair value of warrants included in other revenue.

+ References

No definition available.

+ Details

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

- Definition

Total investments in (A) companies that are accounted for using the equity method and (B) warrants to acquire additional shares of an unconsolidated affiliate that are carried at fair value.

+ References

No definition available.

+ Details

Name:	wtm_InvestmentsInUnconsolidatedAffiliates
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the aggregate value of common shares at quoted market price.

+ References

No definition available.

+ Details

Name:	wtm_InvestmentsInUnconsolidatedAffiliatesAtQuotedMarketPrice
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

Loss resulting from the dilutive effect of dividend payments and issuance of restricted shares by an equity method investee.

+ References

No definition available.

+ Details

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

- Definition

Represents other comprehensive income increase relating to amortizartion of common share basis difference

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_ExtraordinaryItemAxis=wtm_AmortizationofcommonsharebasisdifferenceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ExtraordinaryItemAxis=wtm_ImpairmentofequityinearningsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ExtraordinaryItemAxis=wtm_ImpairmentofnetunrealizedgainslossesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis=wtm_SymetraMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CommonStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_WarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=wtm_EquityinearningsofunconsolidatedaffiliatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=wtm_NetunrealizedgainslossesofunconsolidatedaffiliatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended				6 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jun. 20, 2013
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period				$ 321.4					$ 321.4
Equity in earnings of unconsolidated affiliates, net of tax	12.3	7.0	12.5	13.8	11.7	8.6	7.1	9.2	45.6	36.6	29.9
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates									81.2	(106.4)	62.8
Carrying value at the end of the period	414.4				321.4				414.4	321.4
Income Tax Expense (Benefit)	(8.7)us-gaap_IncomeTaxExpenseBenefit	7.8us-gaap_IncomeTaxExpenseBenefit	23.9us-gaap_IncomeTaxExpenseBenefit	30.3us-gaap_IncomeTaxExpenseBenefit	27.4us-gaap_IncomeTaxExpenseBenefit	8.2us-gaap_IncomeTaxExpenseBenefit	(0.6)us-gaap_IncomeTaxExpenseBenefit	41.6us-gaap_IncomeTaxExpenseBenefit	53.3us-gaap_IncomeTaxExpenseBenefit	76.6us-gaap_IncomeTaxExpenseBenefit	(15.7)us-gaap_IncomeTaxExpenseBenefit
Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period				317.3				381.5	317.3	381.5	273.6		381.5
Equity in earnings of unconsolidated affiliates, net of tax									47.0	37.8	32.3
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates									81.2	(106.4)	62.8
Dividends received									(34.1)	(6.4)	(4.9)
Increase (decrease) in value of warrants										10.8	17.7
Carrying value at the end of the period	411.4				317.3				411.4	317.3	381.5	273.6
Unrealized gains (losses) from Symetra’s fixed maturity portfolio	37.6				(43.6)				37.6	(43.6)
Equity in net unrealized losses increase relating to amortization of common share impairment												136.6
Exercise of warrants										0
Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period				317.3				351.2	317.3	351.2	261.0		351.2
Equity in earnings of unconsolidated affiliates, net of tax									47.0	37.8	32.3
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates									81.2	(106.4)	62.8
Dividends received									(34.1)	(6.4)	(4.9)
Increase (decrease) in value of warrants										0	0
Carrying value at the end of the period	411.4				317.3				411.4	317.3	351.2
Loss from dilutive effect resulting from dividend and issuance of restricted shares									(0.1)	0.2	1.3
Aggregate value of White Mountains common shares of Symetra at quoted market price	462.1								462.1
Share Price	$ 23.05								$ 23.05
Exercise of warrants										41.1
Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates									37.6	(43.6)	62.8
Warrants
Changes in carrying value of investment in Symetra
Increase (decrease) in value of warrants													10.8
Warrants \| Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period				0				30.3	0	30.3	12.6		30.3
Equity in earnings of unconsolidated affiliates, net of tax									0	0	0
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates									0	0	0
Dividends received									0	0	0
Increase (decrease) in value of warrants										10.8	17.7
Carrying value at the end of the period	0				0				0	0	30.3
Share Price											$ 11.49
Exercise of warrants										(41.1)
equity in earnings of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)									3.3	2.8	2.6
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)									5.9	(8.3)	5.1
Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| White Mountains
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)									(2.7)	3.2	(5.1)
impairment of equity in earnings [Member] \| equity in earnings of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax											3.5
impairment of net unrealized gains (losses) [Member] \| Net unrealized gains (losses) of unconsolidated affiliates [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates									12.7	11.8	13.1
Amortization of common share basis difference [Member] \| Common Stock \| Symetra
Changes in carrying value of investment in Symetra
Equity in earnings of unconsolidated affiliates, net of tax									$ 3.0	$ 3.0