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Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Share-based compensation arrangement by share-based payment award |  |  |
Accrued incentive compensation | $ 218.3 | $ 159.0 |
OneBeacon | Performance Shares [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Options nonvested, number before forfeitures (in shares) | 502,428 |  |
Incentive compensation payable before forfeitures | 4.1 |  |
forfeitures | (9,007) |  |
Assumed forfeitures in period amount | (0.1) |  |
Shares outstanding (in shares) | 493,421 |  |
Accrued incentive compensation | 4.0 |  |
Unamortized grant date fair value | 2.9 |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 13.00% | 10.00% | 11.00%
Performance Cycle From 2011 to 2013 | OneBeacon | Performance Shares [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Options nonvested, number before forfeitures (in shares) | 142,138 |  |
Incentive compensation payable before forfeitures | 1.0 |  |
Performance Cycle From 2012 to 2014 | OneBeacon | Performance Shares [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Options nonvested, number before forfeitures (in shares) | 181,290 |  |
Incentive compensation payable before forfeitures | 2.1 |  |
Performance Cycle From 2013 to 2015 | OneBeacon | Performance Shares [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Options nonvested, number before forfeitures (in shares) | 179,000 |  |
Incentive compensation payable before forfeitures | $ 1.0 |  |
Minimum | OneBeacon | Performance Shares [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Performance goal percentage for minimum payout | 6.00% | 3.00% | 4.00%
Maximum | OneBeacon | Performance Shares [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Performance goal percentage for minimum payout | 20.00% | 17.00% | 18.00%
Percentage of performance shares earned | 200.00% | 200.00% | 200.00%

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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Details
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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
Name: | wtm_IncentiveCompensationPayable
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Balance Type: | credit
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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
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- Definition Percentage of performance shares earned + References No definition available. + Details
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- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + References No definition available. + Details
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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
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- Definition Represents the percentage growth in intrinsic business value per share to achieve payment. + References No definition available. + Details
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- Definition Represents the targeted percentage of growth in intrinsic business value per share to achieve 100% payment of outstanding performance shares. + References No definition available. + Details
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