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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, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2010 | Sep. 30, 2010 | Jun. 30, 2010 | Mar. 31, 2010 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |
Net income from continuing operations attributable to White Mountains' common shareholders |  |  |  |  |  |  |  |  | $ 122.6 | $ 93.1 | $ 497.8
Allocation of income for participating unvested restricted common shares |  |  |  |  |  |  |  |  | (1.1) | (1.1) | (4.8)
Dividends paid on participating restricted common shares |  |  |  |  |  |  |  |  | (0.1) | (0.1) | (0.6)
Total allocation to restricted common shares |  |  |  |  |  |  |  |  | (1.2) | (1.2) | (5.4)
Net income attributable to White Mountains' common shareholders, net of restricted share amounts |  |  |  |  |  |  |  |  | 121.4 | 91.9 | 492.4
Undistributed net earnings: |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts |  |  |  |  |  |  |  |  | 121.4 | 91.9 | 492.4
Dividends paid net of restricted common share amounts |  |  |  |  |  |  |  |  | (7.9) | (8.7) | (8.3)
Total undistributed net earnings, net of restricted common share amounts |  |  |  |  |  |  |  |  | 113.5 | 83.2 | 484.1
Net income attributable to White Mountains' common shareholders, net of restricted share amounts |  |  |  |  |  |  |  |  | $ 122.6 | $ 93.1 | $ 497.8
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period (in shares) |  |  |  |  |  |  |  |  | 7,881,000 | 8,548,400 | 8,849,400
Average unvested restricted common shares (in shares) |  |  |  |  |  |  |  |  | (69,400) | (97,300) | (85,300)
Basic earnings per share denominator (in shares) |  |  |  |  |  |  |  |  | 7,811,600 | 8,451,100 | 8,764,100
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period (in shares) |  |  |  |  |  |  |  |  | 7,881,000 | 8,548,400 | 8,849,400
Average unvested restricted common shares (in shares) |  |  |  |  |  |  |  |  | (69,400) | (97,300) | (85,300)
Average outstanding dilutive options to acquire common shares (in shares) |  |  |  |  |  |  |  |  |  | 500 | 1,500
Diluted earnings per share denominator (in shares) |  |  |  |  |  |  |  |  | 7,811,600 | 8,451,600 | 8,765,600
Basic earnings per share (in dollars): |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains' common shareholders | $ 16.54 | $ 1.81 | $ 1.66 | $ (3.82) | $ 9.77 | $ 5.58 | $ 0.21 | $ (4.14) | $ 15.56 | $ 10.89 | $ 56.25
Dividends declared and paid (in dollars per share) |  |  |  |  |  |  |  |  | $ (1.00) | $ (1.00) | $ (1.00)
Undistributed earnings (in dollars per share) |  |  |  |  |  |  |  |  | $ 14.56 | $ 9.89 | $ 55.25
Diluted earnings per share (in dollars) |  |  |  |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share) | $ 16.54 | $ 1.81 | $ 1.66 | $ (3.82) | $ 9.77 | $ 5.58 | $ 0.21 | $ (4.14) | $ 15.56 | $ 10.89 | $ 56.24
Dividends declared and paid (in dollars per share) |  |  |  |  |  |  |  |  | $ (1.00) | $ (1.00) | $ (1.00)
Undistributed earnings (in dollars per share) |  |  |  |  |  |  |  |  | $ 14.56 | $ 9.89 | $ 55.25
Common shares issuable upon exercise of incentive options (in shares) |  |  |  |  |  |  |  |  |  | 1,200 | 5,280
Average strike price of options (in dollars per share) |  |  |  |  | $ 189.31 |  |  |  |  | $ 189.31 | $ 183.03

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- Definition The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Emerging Issues Task Force (EITF) -Number 03-6 -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 Statement of Financial Accounting Standard (FAS) -Number 128 -Paragraph 61 -Subparagraph b -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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- 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
Name: | wtm_DividendsCommonStockCashNetOfRestrictedCommonShareAmounts
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

X
- 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
Name: | wtm_IncomeLossFromContinuingOperatingsExcludingPortionAttributableToNoncontrollingInterestNet
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration