XML 47 R29.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Significant Transactions (Tables) | 3 Months Ended
--- | ---
Mar. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract] |
Reconciliation to the increase in book value | A reconciliation of the gain reported in discontinued operations to the impact to White Mountains's book value is as follows:
Gain from sale of Tranzact reported in discontinued operations |  | $ | 51.9 |
Add back reclassification from continuing operations for the release of a tax valuation allowance |  | 30.2 |
Increase to White Mountains book value from sale of Tranzact |  | $ | 82.1 |

X
- References No definition available. + Details
Name: | us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition [Table Text Block] for Reconciliation of net gain reported in discontinued operations to the increase in book value + References No definition available. + Details
Name: | wtm_ReconciliationtotheincreaseinbookvalueTableTextBlock
Namespace Prefix: | wtm_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration