XML 20 R104.htm IDEA: XBRL DOCUMENT
v3.3.0.814

Subsequent Events (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Subsequent Event [Line Items] |  |  |  |
Other Underwriting Expense | $ 56.5 | $ 51.3 | $ 165.5 | $ 152.6
UNITED STATES | Sale of Sirius [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Stop Loss Limitation |  |  | 45.0 |
Industry Loss Trigger on Industry Loss Warranty |  |  | 15,000.0 |
UNITED STATES | Combination One | Sale of Sirius [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Stop Loss Limitation |  |  | 75.0 |
Industry Loss Trigger on Industry Loss Warranty |  |  | 40,000.0 |
UNITED STATES | Combination Two | Sale of Sirius [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Stop Loss Limitation |  |  | 22.5 |
Industry Loss Trigger on Industry Loss Warranty |  |  | 50,000.0 |
JAPAN | Sale of Sirius [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Stop Loss Limitation |  |  | 25.0 |
Industry Loss Trigger on Industry Loss Warranty |  |  | $ 12,500.0 |

X
- Definition Costs incurred during the period, such as those relating to general administration and policy maintenance that do not vary with and are not primarily related to the acquisition or renewal of insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.7) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 720 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=35755714&loc=d3e28434-158551 + Details
Name: | us-gaap_OtherUnderwritingExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Industry Loss Trigger on Industry Loss Warranty + References No definition available. + Details
Name: | wtm_IndustryLossTriggeronIndustryLossWarranty
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Industry Loss Warranty Limit on Contract + References No definition available. + Details
Name: | wtm_IndustryLossWarrantyLimitonContract
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=wtm_SaleofSiriusMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=wtm_CombinationOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=wtm_CombinationTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=country_JP
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |