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=56944662&loc=d3e3602-108585

Reference 2: 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=35755396&loc=d3e33749-111570

Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 9

-Article 5

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 740

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a),(b)

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

Reference 2: 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 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

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Carrying value of a distribution

+ References

No definition available.

+ Details

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

- 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

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

- References

No definition available.

+ Details

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

- 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_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis=wtm_WhiteMountainsMember
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:

- Details

Name:	us-gaap_ExtraordinaryItemAxis=wtm_AmortizationofcommonsharebasisdifferenceMember
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:

Investments in Unconsolidated Affiliates (Investments in Symetra Table Rollforward) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period			$ 414.4		$ 414.4
Equity in earnings	$ 3.9	$ 7.0			18.0	$ 33.3
Carrying value at the end of the period	392.6				392.6
Income Tax Expense (Benefit)	$ (1.6)	(4.7)			$ 0.8	11.4
Symetra \| Common Stock
Investments in unconsolidated affiliates
Share Price	$ 31.64				$ 31.64
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	$ 397.2	415.5	411.4	$ 317.3	$ 411.4	317.3
Equity in earnings	4.2	7.0			17.5	34.5
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	3.7	(10.7)			(31.8)	64.0
Dividends received	(12.5)	(2.0)			(16.9)	(6.0)
Carrying value at the end of the period	392.6	409.8	$ 397.2	$ 415.5	392.6	409.8
Aggregate value of White Mountains common shares of Symetra at quoted market price	650.6				650.6
Number of Shares Distributed	0.0	0.0			12.4	0.0
White Mountains \| Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio					5.7	20.5
equity in earnings of unconsolidated affiliates \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	0.3	0.5			1.1	2.4
Net unrealized gains (losses) of unconsolidated affiliates \| White Mountains \| Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)					0.2	1.4
Amortization of common share basis difference \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Equity in earnings	0.7	0.7			2.1	2.2
impairment of net unrealized gains (losses) \| Net unrealized gains (losses) of unconsolidated affiliates \| Symetra \| Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	$ 2.9	$ 3.1			$ 8.6	$ 9.4