Significant Transactions Formation of HG and BAM (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended
Sep. 30, 2012
Significant Transactions
Shares remaining under share repurchase authorization	970,496
Percentage of par value of policy reinsured	15.00%
White Mountains [Member]
Significant Transactions
Funds to Capitalized HG Global	$ 594.5
HG Global [Member]
Significant Transactions
Interest income, surplus notes	8.3
Income (Loss) from Continuing Operations before Income Taxes, Foreign	4.8
Surplus Notes	503.0
HG Global [Member] \| Common Stock
Significant Transactions
Ownership interest (as a percent)	89.00%
Percentage of par value of policy reinsured	15.00%
HG Global [Member] \| Preferred Stock
Significant Transactions
Ownership interest (as a percent)	97.00%
Build America Mutual [Member]
Significant Transactions
Interest income, surplus notes	(8.3)
Income (Loss) from Continuing Operations before Income Taxes, Foreign	$ 18.0

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

-Paragraph 18

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 20

-Subparagraph a (1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The portion of earnings or loss from continuing operations before income taxes that is attributable to foreign operations, which is defined as Income or Loss generated from operations located outside the entity's country of domicile.

+ 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)(1)(i))

-URI http://asc.fasb.org/extlink&oid=6881521&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

-Subparagraph 1(i)

-Article 4

+ Details

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

- Definition

The remaining number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 470

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486086&loc=d3e24387-158525

+ Details

Name:	us-gaap_SurplusNotes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Funds used to capitalized HG Global.

+ References

No definition available.

+ Details

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

- Definition

Interest income (expense) earned from surplus notes.

+ References

No definition available.

+ Details

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

- Definition

Percentage of par value of policy reinsured.

+ References

No definition available.

+ Details

Name:	wtm_PercentageOfParValueOfPolicyReinsured
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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