XML 91 R75.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 USD ($) Investment | Dec. 31, 2016 USD ($) Investment | Dec. 31, 2015 USD ($) | Jan. 01, 2017 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | $ 2,547,300,000 | $ 1,543,500,000 |  |
Total realized and unrealized gains (losses) | (157,400,000) | (26,200,000) |  |
Amortization/Accretion | (9,100,000) | (6,100,000) |  |
Purchases | 3,332,500,000 | 4,728,300,000 |  |
Sales | (2,814,600,000) | (3,692,200,000) |  |
Transfers in | 18,700,000 | 70,000,000 |  |
Transfers out | (18,700,000) | (70,000,000) |  |
Fair value investments | 3,208,300,000 | 2,547,300,000 | $ 1,543,500,000 |
Net realized and unrealized investment gains (losses) | 133,300,000 | (27,400,000) | 260,500,000 |
Fixed maturity investments, at fair value |  | 2,081,100,000 |  |
Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 2,639,700,000 |  |  |
Fair value investments | 3,259,100,000 | 2,639,700,000 |  |
Level 1 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 279,500,000 | 789,000,000 |  |
Total realized and unrealized gains (losses) | 82,700,000 | 2,000,000 |  |
Amortization/Accretion | 0 | 100,000 |  |
Purchases | 1,209,300,000 | 1,746,900,000 |  |
Sales | (681,100,000) | (2,258,500,000) |  |
Transfers in | 0 | 0 |  |
Transfers out | 0 | 0 |  |
Fair value investments | 890,400,000 | 279,500,000 | 789,000,000 |
Level 1 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 441,700,000 |  |  |
Fair value investments | 1,041,400,000 | 441,700,000 |  |
Level 2 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 2,093,800,000 | 585,600,000 |  |
Total realized and unrealized gains (losses) | 69,600,000 | (8,000,000) |  |
Amortization/Accretion | (9,100,000) | (6,200,000) |  |
Purchases | 2,007,900,000 | 2,868,800,000 |  |
Sales | (2,070,300,000) | (1,416,400,000) |  |
Transfers in | 18,700,000 | 70,000,000 |  |
Transfers out | 0 | 0 |  |
Fair value investments | 2,105,400,000 | 2,093,800,000 | 585,600,000 |
Level 2 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 2,106,600,000 |  |  |
Fair value investments | 2,130,500,000 | 2,106,600,000 |  |
Level 3 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 91,400,000 |  |  |
Fair value investments | 87,200,000 | 91,400,000 |  |
Star & Shield Insurance Exchange |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates | (5,200,000) |  |  |
Fixed maturity investments, at fair value |  | 6,600,000.0 |  |
Star & Shield Insurance Exchange | Level 1 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates | 0 |  |  |
Star & Shield Insurance Exchange | Level 2 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates | (5,200,000) |  |  |
Fixed maturity investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 2,087,700,000 |  |  |
Fair value investments | 2,129,700,000 | 2,087,700,000 |  |
Fixed maturity investments | Level 1 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 101,500,000 |  |  |
Fair value investments | 296,500,000 | 101,500,000 |  |
Fixed maturity investments | Level 2 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 1,986,200,000 |  |  |
Fair value investments | 1,833,200,000 | 1,986,200,000 |  |
Fixed maturity investments | Level 3 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 0 | 0 |  |
Total realized and unrealized gains (losses) | 0 | 100,000 |  |
Amortization/Accretion | 0 | 0 |  |
Purchases | 31,200,000 | 69,900,000 |  |
Sales | (12,500,000) |  |  |
Transfers in | 0 | 0 |  |
Transfers out | 18,700,000 | (70,000,000) |  |
Fair value investments | $ 0 | $ 0 | 0 |
Number of investments | Investment | 3 | 3 |  |
Fixed maturity investments | Level 3 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | $ 0 |  |  |
Fair value investments | 0 | $ 0 |  |
Fixed maturity investments | Star & Shield Insurance Exchange |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 6,600,000 |  |  |
Fair value investments |  | 6,600,000 |  |
Fixed maturity investments, at fair value |  | 6,600,000 | 9,500,000 |
Fixed maturity investments | Star & Shield Insurance Exchange | Level 3 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates | 0 |  |  |
Other long-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 91,400,000 |  |  |
Fair value investments | 87,200,000 | 91,400,000 |  |
Other long-term investments | Level 1 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 0 |  |  |
Fair value investments | 0 | 0 |  |
Other long-term investments | Level 2 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 0 |  |  |
Fair value investments | 0 | 0 |  |
Other long-term investments | Level 3 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 91,400,000 | 103,600,000 |  |
Total realized and unrealized gains (losses) | (15,300,000) | (14,300,000) |  |
Amortization/Accretion | 0 | 0 |  |
Purchases | 13,100,000 | 2,200,000 |  |
Sales | (2,000,000) | (100,000) |  |
Transfers in | 0 | 0 |  |
Transfers out | 0 | 0 |  |
Fair value investments | 87,200,000 | 91,400,000 | 103,600,000 |
Other long-term investments | Level 3 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 91,400,000 |  |  |
Fair value investments | 87,200,000 | 91,400,000 |  |
Other long-term investments | Star & Shield Insurance Exchange | Level 3 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates | 0 |  |  |
Total hedge and private equity funds included in other long-term investments | Level 3 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 82,600,000 | 65,300,000 |  |
Total realized and unrealized gains (losses) | 20,400,000 | (6,000,000) |  |
Amortization/Accretion | 0 | 0 |  |
Purchases | 71,000,000 | 40,500,000 |  |
Sales | (48,700,000) | (17,200,000) |  |
Transfers in | 0 | 0 |  |
Transfers out | 0 | 0 |  |
Fair value investments | 125,300,000 | 82,600,000 | 65,300,000 |
Total hedge and private equity funds included in other long-term investments | Star & Shield Insurance Exchange | Level 3 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates | 0 |  |  |
Short-term investments |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net realized and unrealized investment gains (losses) | (300,000.0) |  |  |
Short-term investments | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 175,000,000 | 142,000,000 |  |
Fair value investments | 176,100,000 | 175,000,000 | 142,000,000 |
Short-term investments | Level 1 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 162,300,000 |  |  |
Fair value investments | 151,000,000 | 162,300,000 |  |
Short-term investments | Level 2 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 12,700,000 |  |  |
Fair value investments | 25,100,000 | 12,700,000 |  |
Short-term investments | Level 3 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 0 |  |  |
Fair value investments | 0 | 0 |  |
Short-term investments | Star & Shield Insurance Exchange |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | 100,000 |  |  |
Fair value investments |  | 100,000 |  |
Short-term investments | Star & Shield Insurance Exchange | Level 1 Inputs |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Short-term investments, at amortized cost (which approximates fair value) |  | 100,000 | $ 0 | $ 100,000
Foreign Exchange Forward [Member] |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Net realized and unrealized investment gains (losses) | (23,800,000.0) | (1,200,000.0) |  |
Foreign Exchange Forward [Member] | Level 3 Inputs | Fair value measured on a recurring basis |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Fair value investments | (1,200,000) |  |  |
Fair value investments | $ (3,700,000) | $ (1,200,000) |  |

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=98514028&loc=d3e3602-108585 + Details
Name: | us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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 -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 3: 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=82895926&loc=d3e24584-111560 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 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 6: 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=75030427&loc=d3e22054-111558 Reference 7: 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 + Details
Name: | us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27476-111563 + Details
Name: | us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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 Amount of investments in debt securities 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 2 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562 Reference 2: 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 Reference 3: 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 4: 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=82896770&loc=d3e26610-111562 + Details
Name: | us-gaap_TradingSecuritiesDebt
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 Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates + References No definition available. + Details
Name: | wtm_NetChangeInInvestmentsRelatedToPurchasesAndSalesOfConsolidatedunconsolidatedAffiliates
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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

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_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
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: |

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

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