XML 52 R100.htm IDEA: XBRL DOCUMENT
v2.4.0.6

Subsequent Event (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Sep. 30, 2012 Months | Oct. 16, 2012
Subsequent Event [Line Items] |  |
Hagerty written premiums number of months | 12 |
Significant Acquisitions and Disposals, Gain (Loss) on Sale or Disposal, Net of Tax |  | $ 15
Hagerty Insurance Agency [Member] |  |
Subsequent Event [Line Items] |  |
Direct Premiums Written | 179 |
Hagerty percentage of direct written premiums | 16.00% |

X
- Definition Total consideration paid and payable by direct policyholders for insurance contracts sold by the Entity during the period. This amount does not include premium assumed from or ceded to other insurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -Subparagraph a, b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
Name: | us-gaap_DirectPremiumsWritten
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

X
- Definition Gain (loss) estimated from transaction, net of tax. + References No definition available. + Details
Name: | wtm_GainLossEstimatedFromTransactionNetOfTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Hagerty percentage of direct written premiums. + References No definition available. + Details
Name: | wtm_HagertyPercentageOfDirectWrittenPremiums
Namespace Prefix: | wtm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Hagerty written premiums number of months. + References No definition available. + Details
Name: | wtm_HagertyWrittenPremiumsNumberOfMonths
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration