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Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 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
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net (loss) income attributable to White Mountains’ common shareholders |  |  |  |  |  |  |  |  | $ 322.4 |  | $ 146.3 |  | $ 116.6 |
Allocation of income for participating unvested restricted common shares |  |  |  |  |  |  |  |  | (4.3) |  | (1.2) |  | (1.3) |
Dividends declared on participating restricted common shares (1) |  |  |  |  |  |  |  |  | (0.1) | [1] | (0.1) | [1] | (0.1) | [1]
Total allocation to restricted common shares |  |  |  |  |  |  |  |  | (4.4) |  | (1.3) |  | (1.4) |
Net income attributable to White Mountains’ common shareholders, net of restricted share amounts |  |  |  |  |  |  |  |  | 318.0 |  | 145.0 |  | 115.2 |
Undistributed net earnings: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts |  |  |  |  |  |  |  |  | 318.0 |  | 145.0 |  | 115.2 |
Dividends declared net of restricted common share amounts (1) |  |  |  |  |  |  |  |  | (6.5) | [1] | (7.9) | [1] | (8.7) | [1]
Total undistributed net earnings, net of restricted common share amounts |  |  |  |  |  |  |  |  | $ 311.5 |  | $ 137.1 |  | $ 106.5 |
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 6,799,800 |  | 7,881,000 |  | 8,548,400 |
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (91,100) | [2] | (69,400) | [2] | (97,300) | [2]
Basic earnings per share denominator |  |  |  |  |  |  |  |  | 6,708,700 |  | 7,811,600 |  | 8,451,100 |
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 6,799,800 |  | 7,881,000 |  | 8,548,400 |
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (91,100) | [2] | (69,400) | [2] | (97,300) | [2]
Average outstanding dilutive options to acquire common shares |  |  |  |  |  |  |  |  | 0 | [3] | 0 | [3] | 500 | [3]
Diluted earnings (loss) per share denominator |  |  |  |  |  |  |  |  | 6,708,700 |  | 7,811,600 |  | 8,451,600 |
Basic earnings per share (in dollars): |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’ common shareholders | $ 10.36 | $ 19.11 | $ 3.85 | $ 13.85 | $ 20.03 | $ 1.80 | $ 1.78 | $ (4.33) | $ 47.41 |  | $ 18.56 |  | $ 13.63 |
Dividends declared and paid |  |  |  |  |  |  |  |  | $ (1.00) |  | $ (1.00) |  | $ (1.00) |
Undistributed earnings |  |  |  |  |  |  |  |  | $ 46.41 |  | $ 17.56 |  | $ 12.63 |
Diluted earnings per share (in dollars) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’ common shareholders | $ 10.36 | $ 19.11 | $ 3.85 | $ 13.85 | $ 20.03 | $ 1.80 | $ 1.78 | $ (4.33) | $ 47.41 |  | $ 18.56 |  | $ 13.63 |
Dividends declared and paid |  |  |  |  |  |  |  |  | $ (1.00) |  | $ (1.00) |  | $ (1.00) |
Undistributed earnings |  |  |  |  |  |  |  |  | $ 46.41 |  | $ 17.56 |  | $ 12.63 |
Incremental common shares attributable to share-based payment arrangements |  |  |  |  |  |  |  |  |  |  |  |  | 1,200 |
Average strike price of options (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  | $ 189 |
[1] | (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities
[2] | Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 11).
[3] | The diluted earnings per share denominator for the year ended December 31, 2010 includes 1,200 common shares issuable upon exercise of incentive options at an average strike price of $189.31 per common share. The non-qualified options were not included in the diluted earnings per share denominator for any of the periods presented as their inclusion would be anti-dilutive.

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- Definition Number of shares of restricted stock determined by relating the portion of time within a reporting period that restricted shares have been outstanding to the total time in that period. Restricted shares are subject to sales, contractual, regulatory or other restrictions that prevent or inhibit the holder from freely disposing of them before the restriction ends. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2646-109256 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 128 -Paragraph 10 -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
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Balance Type: | na
Period Type: | duration

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- Definition Represents the total amount allocated to restricted shares including dividend on restricted stock. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Equity impact of common stock cash dividends declared by the entity during the period, net of restricted share amounts. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents the portion of income (loss) allocated to participating unvested restricted common shares. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Represents Net income from continuing operations attributable to White Mountains common shareholders. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents Net income from continuing operations attributable to White Mountains common shareholders, net of allocation to restricted shares and dividends paid on restricted shares. + References No definition available. + Details
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