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Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details) - $ / shares | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Performance Shares |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number | 125,000 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding Amended, Exercise Price | $ 742 |  |
Qualified Plan [Member] |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Percentage of common shares outstanding | 3.00% | 3.00% |
Eligible contribution percentage | 3.00% | 2.00% | 4.00%

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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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

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- Definition This element represents the amended exercise price for the non-qualified options. + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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