XML 72 R60.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Millions | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | $ 2,547.3 | $ 1,543.5 |  |
Net realized and unrealized gains (losses) | (83.0) | 16.3 |  |
Amortization/Accretion | (4.7) | (2.0) |  |
Purchases | 1,516.4 | 3,066.7 |  |
Sales | 1,503.0 | 2,019.9 |  |
Transfers in | 11.1 | 0.0 |  |
Transfers out | (11.1) | 0.0 |  |
Ending balance | 2,633.8 | 2,604.6 |  |
Short-term investments, at amortized cost (which approximates fair value) | 71.6 |  | $ 174.9 |
Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 279.5 | 789.0 |  |
Net realized and unrealized gains (losses) | 30.4 | 3.6 |  |
Amortization/Accretion | 0.0 | 0.1 |  |
Purchases | 665.0 | 1,312.9 |  |
Sales | 243.9 | 1,809.9 |  |
Transfers in | 0.0 | 0.0 |  |
Transfers out | 0.0 | 0.0 |  |
Ending balance | 731.0 | 295.7 |  |
Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 2,093.8 | 585.6 |  |
Net realized and unrealized gains (losses) | 39.0 | 13.7 |  |
Amortization/Accretion | (4.7) | (2.1) |  |
Purchases | 770.7 | 1,668.7 |  |
Sales | 1,255.0 | 207.0 |  |
Transfers in | 11.1 | 0.0 |  |
Transfers out | 0.0 | 0.0 |  |
Ending balance | 1,649.7 | 2,058.9 |  |
Fixed maturity investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 0.0 | 0.0 |  |
Net realized and unrealized gains (losses) | 0.1 | 0.1 |  |
Amortization/Accretion | 0.0 | 0.0 |  |
Purchases | 25.6 | 67.9 |  |
Sales | 0.5 |  |  |
Transfers in | 0.0 | 0.0 |  |
Transfers out | 11.1 | 0.0 |  |
Ending balance | 14.1 | 68.0 |  |
Other long-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 91.4 | 103.6 |  |
Net realized and unrealized gains (losses) | (1.5) | 1.6 |  |
Amortization/Accretion | 0.0 | 0.0 |  |
Purchases | 2.6 | 2.0 |  |
Sales | 2.0 |  |  |
Transfers in | 0.0 | 0.0 |  |
Transfers out | 0.0 | 0.0 |  |
Ending balance | 90.5 | 107.2 |  |
Hedge and private equity funds included in other long-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 82.6 | 65.3 |  |
Net realized and unrealized gains (losses) | 15.0 | (2.7) |  |
Amortization/Accretion | 0.0 | 0.0 |  |
Purchases | 52.5 | 15.2 |  |
Sales | 1.6 | 3.0 |  |
Transfers in | 0.0 | 0.0 |  |
Transfers out | 0.0 | 0.0 |  |
Ending balance | 148.5 | 74.8 |  |
Fair value measured on a recurring basis |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 2,639.7 |  |  |
Ending balance | 2,556.9 |  |  |
Fair value measured on a recurring basis | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 441.7 |  |  |
Ending balance | 786.7 |  |  |
Fair value measured on a recurring basis | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 2,106.6 |  |  |
Ending balance | 1,665.6 |  |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 91.4 |  |  |
Ending balance | 104.6 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 2,087.7 |  |  |
Ending balance | 1,566.9 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 101.5 |  |  |
Ending balance | 49.7 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 1,986.2 |  |  |
Ending balance | 1,503.1 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 0.0 |  |  |
Ending balance | 14.1 |  |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 91.4 |  |  |
Ending balance | 90.5 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 0.0 |  |  |
Ending balance | 0.0 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 0.0 |  |  |
Ending balance | 0.0 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 91.4 |  |  |
Ending balance | 90.5 |  |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 175.0 |  |  |
Ending balance | 71.6 |  |  |
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 162.3 |  |  |
Ending balance | 55.7 |  |  |
Short-term investments, at amortized cost (which approximates fair value) |  | 307.3 | 175.0 | $ 142.0
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 12.7 |  |  |
Ending balance | 15.9 |  |  |
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 0.0 |  |  |
Ending balance | 0.0 |  |  |
Star & Shield Insurance Exchange | Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Short-term investments, at amortized cost (which approximates fair value) |  | 0.3 | 0.1 | 0.1
Star & Shield Insurance Exchange |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Trading securities debt reclassified to assets held-for-sale |  | $ 9.2 | 6.6 | $ 9.5
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Derivative, fair value, net | (12.5) |  | $ (1.2) |
Star & Shield Insurance Exchange |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Sales | 5.2 |  |  |
Star & Shield Insurance Exchange | Level 1 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Sales | 0.0 |  |  |
Star & Shield Insurance Exchange | Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Sales | 5.2 |  |  |
Star & Shield Insurance Exchange | Fixed maturity investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | 6.6 |  |  |
Star & Shield Insurance Exchange | Fixed maturity investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Sales | 0.0 |  |  |
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Sales | 0.0 |  |  |
Star & Shield Insurance Exchange | Hedge and private equity funds included in other long-term investments | Level 3 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Sales | 0.0 |  |  |
Star & Shield Insurance Exchange | Short-term investments |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Beginning balance | $ 0.1 |  |  |

X
- Definition The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
Name: | us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611 + Details
Name: | us-gaap_DerivativeFairValueOfDerivativeNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + 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 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(5)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: 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(6)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 Reference 6: 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(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558 Reference 8: 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)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 + Details
Name: | us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current. + 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(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: 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)(g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: 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(5)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: 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(6)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 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_ShortTermInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents the amount of the total realized and unrealized (holding) gains or losses for the period which are included in the statement of income. + References No definition available. + Details
Name: | wtm_FairValueAssetsMeasuredOnRecurringBasisGainLossIncludedInEarnings
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
Name: | wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersIn
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
Name: | wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersOut
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents the fair value of trading debt securities re-classified to assets held for sale. + References No definition available. + Details
Name: | wtm_TradingSecuritiesDebtReclassifiedToAssetsHeldForSale
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

X
- Details
Name: | dei_LegalEntityAxis=wtm_StarShieldInsuranceExchangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=wtm_StarShieldInsuranceExchangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=wtm_StarShieldInsuranceExchangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |