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Investments in Unconsolidated Affiliates (Financial Information) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended |  | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 Symetra | Dec. 31, 2011 Symetra | Dec. 31, 2010 Symetra | Dec. 31, 2009 Symetra | Apr. 30, 2007 Pentelia Capital Management | Dec. 31, 2012 Pentelia Capital Management | Dec. 31, 2011 Pentelia Capital Management | Dec. 31, 2010 Pentelia Capital Management | Dec. 31, 2010 Delos | Aug. 31, 2006 Delos | Dec. 31, 2012 Hamer and Bri-Mar
Symetra balance sheet data; [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total investments |  |  |  |  |  |  |  |  |  |  |  | $ 27,556.4 |  | $ 26,171.7 |  |  |  |  |  |  |  |  |  |  |  |
Separate account assets |  |  |  |  |  |  |  |  |  |  |  | 807.7 |  | 795.8 |  |  |  |  |  |  |  |  |  |  |  |
Total assets |  |  |  |  |  |  |  |  |  |  |  | 29,460.9 | [1] | 28,183.3 | [1] |  |  |  |  |  |  |  |  |  |  |
Policyholder liabilities |  |  |  |  |  |  |  |  |  |  |  | 23,735.2 |  | 23,140.6 |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  |  |  |  | 449.4 |  | 449.2 |  |  |  |  |  |  |  |  |  |  |  |
Separate account liabilities |  |  |  |  |  |  |  |  |  |  |  | 807.7 |  | 795.8 |  |  |  |  |  |  |  |  |  |  |  |
Total liabilities |  |  |  |  |  |  |  |  |  |  |  | 25,830.8 | [1] | 25,068.4 | [1] |  |  |  |  |  |  |  |  |  |  |
Common shareholders’ equity |  |  |  |  |  |  |  |  |  |  |  | 3,630.1 | [1] | 3,114.9 | [1] |  |  |  |  |  |  |  |  |  |  |
Symetra income statement data: [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  |  |  |  | 605.0 |  | 540.5 |  | 473.0 |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  |  |  |  | 1,275.2 |  | 1,270.9 |  | 1,199.4 |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  |  |  |  | 2,101.2 | [2] | 1,999.3 | [2] | 1,878.8 | [2] |  |  |  |  |  |  |  |  |
Policy benefits |  |  |  |  |  |  |  |  |  |  |  | 1,371.8 |  | 1,307.3 |  | 1,234.6 |  |  |  |  |  |  |  |  |  |
Total expenses |  |  |  |  |  |  |  |  |  |  |  | 1,831.1 | [2] | 1,726.1 | [2] | 1,600.5 | [2] |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  |  |  |  | 205.4 | [2] | 195.8 | [2] | 193.8 | [2] |  |  |  |  |  |  |  |  |
Comprehensive net income |  |  |  |  |  |  |  |  |  |  |  | 549.3 |  | 785.5 |  | 679.0 |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |  |  |  | 33.00% |  |  |  |  | 18.00% |
Payments to acquire investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.6 |  |  |  |  | 32.0 |
Equity in earnings (losses) of unconsolidated affiliates | 5.5 | 7.7 | 6.5 | 10.2 | (36.4) | 1.5 | 7.9 | 6.8 | 29.9 | (20.2) | 9.9 | 32.3 | [3],[4],[5] | 28.2 | [3],[4] | 11.1 | [3],[4],[6] |  |  |  | (1.3) | (0.2) | 0.5 |  |  | 0.4
Investments in unconsolidated affiliates | 387.9 |  |  |  | 275.3 |  |  |  | 387.9 | 275.3 |  | 381.5 | [7],[8] | 273.6 | [7] | 387.5 | [7] | 307.7 | [7] |  | 0.5 | 1.7 |  |  |  |
Net sales proceeds |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.7 | 138.8 |
Gain (loss) on dsposal |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (10.9) | $ 14.0 |
[1] | 2011 balances have been restated for the effect of Symetra’s adoption of ASU 2010-26
[2] | Amounts for the years ended December 31, 2011 and 2010 have been restated for the effect of Symetra’s adoption of ASU 2010-26.
[3] | Equity in earnings for the years end December 31, 2012, 2011 and 2010 includes $1.3, $1.0, and $0.8 loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra
[4] | Equity in earnings for the years end December 31, 2012, 2011 and 2010 excludes tax expense of $2.6, $2.3, and $1.4
[5] | Equity in earnings includes $3.5 increase relating to the pre-tax amortization of Symetra common share impairment from December 31, 2012.
[6] | Includes a $17.9 loss from the dilutive effect of Symetra’s public offering
[7] | Includes White Mountains’ equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $62.8, $0, and $63.7 as of December 31, 2012, 2011 and 2010, which excludes tax expense of $5.1, $0 and $5.2
[8] | The aggregate value of White Mountains’ investment in common shares of Symetra was $225.9 based upon the quoted market price of $12.98 per share at December 31, 2012.

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- Definition Premiums earned on the income statement for all insurance and reinsurance contracts after subtracting any amounts ceded to another insurer and adding premiums assumed from other insurers reported by an equity method investment of the entity. + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentSummarizedFinancialInformationPremiumsEarnedNet
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of separate account assets reported by an equity method investment of the entity. + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountAssets
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of separate account liabilities reported by an equity method investment of the entity. + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentSummarizedFinancialInformationSeparateAccountsLiability
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of the total expenses reported by an equity method investment of the entity. + References No definition available. + Details
Name: | wtm_EquityMethodInvestmentSummarizedFinancialInformationTotalExpenses
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- Details
Name: | wtm_SymetraBalanceSheetDataAbstract
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | wtm_SymetraIncomeStatementDataAbstract
Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration