XML 109 R89.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) $ in Millions | Dec. 31, 2015 USD ($) fund | Dec. 31, 2014 USD ($)
--- | --- | ---
Private equity funds |  |
Investment securities |  |
Number of Investments Held | fund | 22 |
Trading Securities, Other | $ 90.8 |
Hedge funds |  |
Investment securities |  |
Number of Investments Held | fund | 5 |
Trading Securities, Other | $ 37.0 |
Total hedge and private equity funds included in other long-term investments |  |
Investment securities |  |
Trading Securities, Other | 127.8 | $ 178.5
Fair value unfunded commitments | 82.2 | 64.3
Other Long-term Investments Largest Single Investment | 22.7 | 22.0
Private equity funds | Private equity funds |  |
Investment securities |  |
Trading Securities, Other | 90.8 | 104.1
Fair value unfunded commitments | 82.2 | 64.3
Private equity funds | Multi-sector |  |
Investment securities |  |
Trading Securities, Other | 14.8 | 14.5
Fair value unfunded commitments | 2.1 | 2.2
Private equity funds | Energy infrastructure & services |  |
Investment securities |  |
Trading Securities, Other | 20.7 | 33.1
Fair value unfunded commitments | 3.4 | 4.8
Private equity funds | Real estate |  |
Investment securities |  |
Trading Securities, Other | 0.4 | 1.7
Fair value unfunded commitments | 0.1 | 0.1
Private equity funds | Private equity secondaries |  |
Investment securities |  |
Trading Securities, Other | 4.4 | 5.7
Fair value unfunded commitments | 2.1 | 2.1
Private equity funds | Manufacturing/Industrial |  |
Investment securities |  |
Trading Securities, Other | 24.9 | 23.2
Fair value unfunded commitments | 2.5 | 7.3
Private equity funds | Healthcare |  |
Investment securities |  |
Trading Securities, Other | 3.8 | 3.1
Fair value unfunded commitments | 0.4 | 1.4
Private equity funds | Insurance |  |
Investment securities |  |
Trading Securities, Other | 2.0 | 2.1
Fair value unfunded commitments | 41.3 | 41.3
Private equity funds | Aerospace/Defense/Government |  |
Investment securities |  |
Trading Securities, Other | 19.8 | 20.7
Fair value unfunded commitments | 30.3 | 5.1
Hedge funds | Hedge funds |  |
Investment securities |  |
Trading Securities, Other | 37.0 | 74.4
Fair value unfunded commitments | 0.0 | 0.0
Hedge funds | Long/short credit & distressed |  |
Investment securities |  |
Trading Securities, Other | 0.0 | 8.4
Fair value unfunded commitments | 0.0 | 0.0
Hedge funds | Long/short equity REIT |  |
Investment securities |  |
Trading Securities, Other | 20.6 | 20.4
Fair value unfunded commitments | 0.0 | 0.0
Hedge funds | Other hedge funds |  |
Investment securities |  |
Trading Securities, Other | 3.6 | 15.7
Fair value unfunded commitments | 0.0 | 0.0
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial [Member] |  |
Investment securities |  |
Trading Securities, Other | 12.8 | 29.9
Fair value unfunded commitments | $ 0.0 | $ 0.0

X
- Definition The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258 + Details
Name: | us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of other investments classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26626-111562 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.(a),1(c),(d),(e),(f),(g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_TradingSecuritiesOther
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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_PrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_HedgeFundsMember
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: |

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

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

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

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

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

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

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

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

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

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

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

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

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

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