XML 74 R62.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |
Payments to acquire businesses and interest in affiliates |  | $ 0.0 | $ 8.1
Oversee.net |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |
Payments to acquire businesses and interest in affiliates | $ 3.9 |  |

X
- 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
Name: | us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cash outflow associated with the acquisition of a controlling interest in another entity or an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3213-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 + Details
Name: | us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=wtm_Oversee.netMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |