XML 13 R54.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Investment Securities (Hedge Funds and Private Equity Funds) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Other Investments | $ 402.6 us-gaap_OtherInvestments | $ 407.0 us-gaap_OtherInvestments
Hedge funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Number of investments | 12 wtm_NumberOfInvestmentsHeld / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember |
Private equity funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Number of investments | 31 wtm_NumberOfInvestmentsHeld / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsMember |
Hedge and private equity funds included in other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Largest single fund investment | $ 29.4 wtm_OtherLongTermInvestmentsLargestSingleInvestment / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsAndPrivateEquityFundsMember |

X
- Definition Other investments not otherwise specified in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 12 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_OtherInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

X
- Definition Number of investments held. + References No definition available. + Details
Name: | wtm_NumberOfInvestmentsHeld
Namespace Prefix: | wtm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The largest investment which the entity has made in a single fund. + References No definition available. + Details
Name: | wtm_OtherLongTermInvestmentsLargestSingleInvestment
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_HedgeFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=wtm_HedgeFundsAndPrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |