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.

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

- 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=77997519&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=77997519&loc=d3e13495-108611

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

- 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

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_OtherLongTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=wtm_ForeignCurrencyonCashandOpenTradesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	dei_LegalEntityAxis=wtm_StarShieldInsuranceExchangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 3,208,300,000	$ 2,547,300,000
Net realized and unrealized (losses) gains	(45,400,000)	39,100,000
Amortization/Accretion	(1,200,000)	(2,500,000)
Purchases	692,600,000	498,900,000
Sales	(1,321,800,000)	(466,800,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	2,532,500,000	2,610,800,000
Net realized and unrealized investment (losses) gains	(53,700,000)	36,300,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	890,400,000	279,500,000
Net realized and unrealized (losses) gains	(14,300,000)	13,100,000
Amortization/Accretion	0	0
Purchases	238,900,000	115,200,000
Sales	(240,700,000)	(76,000,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	874,300,000	331,800,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,105,400,000	2,093,800,000
Net realized and unrealized (losses) gains	(26,000,000)	18,400,000
Amortization/Accretion	(1,200,000)	(2,500,000)
Purchases	407,700,000	351,100,000
Sales	(1,080,800,000)	(387,600,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	1,405,100,000	2,068,000,000
Fixed maturity investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0	0
Net realized and unrealized (losses) gains	0	100,000
Amortization/Accretion	0	0
Purchases	0	11,000,000
Sales	0
Transfers in	0	0
Transfers out	0	0
Ending balance	0	11,100,000
Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	87,200,000	91,400,000
Net realized and unrealized (losses) gains	(8,700,000)	200,000
Amortization/Accretion	0	0
Purchases	44,000,000	200,000
Sales	0	(2,000,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	122,500,000	89,800,000
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	125,300,000	82,600,000
Net realized and unrealized (losses) gains	3,600,000	7,300,000
Amortization/Accretion	0	0
Purchases	2,000,000	21,400,000
Sales	(300,000)	(1,200,000)
Transfers in	0	0
Transfers out	0	0
Ending balance	130,600,000	110,100,000
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains	600,000.0
Foreign Exchange Forward
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains	3,500,000.0	2,800,000.0
Foreign Currency on Cash and Open Trades
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains	4,200,000.0
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	3,259,100,000
Ending balance	3,164,900,000
Fair value measured on a recurring basis \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	1,041,400,000
Ending balance	1,611,600,000
Fair value measured on a recurring basis \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,130,500,000
Ending balance	1,430,800,000
Fair value measured on a recurring basis \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	87,200,000
Ending balance	122,500,000
Fair value measured on a recurring basis \| Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,129,700,000
Ending balance	1,333,800,000
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	296,500,000
Ending balance	197,700,000
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	1,833,200,000
Ending balance	1,136,100,000
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
Ending balance	0
Fair value measured on a recurring basis \| Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	87,200,000
Ending balance	122,500,000
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
Ending balance	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
Ending balance	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	87,200,000
Ending balance	122,500,000
Fair value measured on a recurring basis \| Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	176,100,000
Short-term investments, at amortized cost (which approximates fair value)			$ 763,000,000
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	151,000,000
Ending balance	737,300,000
Short-term investments, at amortized cost (which approximates fair value)		138,200,000	176,100,000	$ 175,000,000
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	25,100,000
Ending balance	25,700,000
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
Ending balance	$ 0
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		(4,100,000)	$ (3,700,000)	(1,200,000)
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		5,200,000
Star & Shield Insurance Exchange \| Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		0
Star & Shield Insurance Exchange \| Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		5,200,000
Star & Shield Insurance Exchange \| Fixed maturity investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		0
Star & Shield Insurance Exchange \| Other long-term investments \| Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Deconsolidation of SSIE		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]
Deconsolidation of SSIE		$ 0
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities debt reclassified to assets held-for-sale				6,600,000
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)				$ 100,000