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Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Qualified Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Eligible contribution percentage | 4.00% | 3.00% | 2.00%
Percentage of common shares outstanding | 3.00% | 3.00% |
One Beacon Employee Stock Ownership Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Defined Contribution Plan, Cost Recognized | $ 6.3 | $ 4.8 | $ 6.3

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the variable contribution amounts earned by eligible participants of the KSOP as a percentage of salary. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the percentage of total common shares outstanding owned by the plans. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration