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Income Taxes (Details 3) (USD $) In Millions, unless otherwise specified | Dec. 31, 2011 | Dec. 31, 2010
--- | --- | ---
Deferred income tax assets related to: |  |
Non-U.S. net operating loss carryforwards | $ 504.0 | $ 545.4
Loss reserve discount | 114.2 | 142.4
U.S. federal and state net operating loss carryforwards | 102.7 | 103.6
Incentive compensation | 43.4 | 36.9
Unearned premiums | 41.2 | 37.3
U.S. tax credit carryforwards | 13.5 | 12.0
Deferred compensation | 11.4 | 13.1
Loss on sale of AutoOne | 10.4 |
Pension and benefit accruals | 8.0 | 4.3
Accrued interest | 3.5 | 0.5
Investment basis differences | 1.6 | 7.3
Fixed assets | (0.1) | 4.8
Other items | 9.5 | 14.1
Total gross deferred income tax assets | 863.3 | 921.7
Less: valuation allowances | (232.7) | (385.8)
Total net deferred income tax assets | 630.6 | 535.9
Deferred income tax liabilities related to: |  |
Safety reserve | 369.6 | 378.5
Deferred acquisition costs | 48.8 | 41.5
Net unrealized investment gains | 34.8 | 55.8
Purchase accounting | 0.7 | 2.1
Intangible assets | (0.1) | 0.1
Other items | 5.4 | (0.8)
Total deferred income tax liabilities | 459.2 | 477.2
Net deferred tax asset (liability) | $ 171.4 | $ 58.7

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- Definition The amount as of the balance sheet date of the estimated future tax effects attributable to the difference between the methods used to account for deferred acquisition costs for tax purposes and under generally accepted accounting principles which will increase future taxable income when such difference reverses. + References No definition available. + Details
Name: | wtm_DeferredTaxLiabilitiesDeferredExpenseDeferredAcquisitionCost
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Represents the amount as of the balance sheet date of the estimated future tax effects attributable to the difference between the tax basis of purchase accounting methodology and the basis of invested assets computed in accordance with generally accepted accounting principles. + References No definition available. + Details
Name: | wtm_DeferredTaxLiabilitiesPurchaseAccountingMethodology
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount as of the balance sheet date of the estimated future tax effects attributable to the difference between the methods used to account for safety reserve for tax purposes and under generally accepted accounting principles which will increase future taxable income when such difference reverses. + References No definition available. + Details
Name: | wtm_DeferredTaxLiabilitiesSafetyReserve
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant