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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

+ Details

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120400017&loc=d3e572229-122910

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates.

+ References

No definition available.

+ Details

Name:	wtm_EquityInUnrealizedGainsLossesFromInvestmentsInUnconsolidatedAffiliates
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

Securities, Realized and Unrealized Gain (Loss)

+ References

No definition available.

+ Details

Name:	wtm_SecuritiesRealizedandUnrealizedGainLoss
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
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_FinancialInstrumentAxis=us-gaap_OtherLongTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=wtm_KuduInvestmentManagementLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	$ 2,328.7	$ 2,746.0	$ 2,328.7
Net realized and unrealized gains (losses)		302.4	347.5
Amortization/Accretion		(3.0)	(1.0)
Purchases		563.6	750.4
Sales		(1,075.6)	(856.2)
Transfers in		0.0	82.6
Transfers out		0.0	(82.6)
Ending balance		2,533.4	2,569.4
Level 1 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	842.6	780.0	842.6
Net realized and unrealized gains (losses)		29.2	121.8
Amortization/Accretion		0.0	0.2
Purchases		128.8	101.4
Sales		(642.7)	(362.3)
Transfers in		0.0	0.0
Transfers out		0.0	0.0
Ending balance		295.3	703.7
Level 2 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	1,160.5	1,109.6	1,160.5
Net realized and unrealized gains (losses)		12.5	66.6
Amortization/Accretion		(3.0)	(1.2)
Purchases		316.2	331.3
Sales		(368.5)	(465.2)
Transfers in		0.0	0.0
Transfers out		0.0	0.0
Ending balance		1,066.8	1,092.0
Common equity securities \| Level 3 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance		0.1
Net realized and unrealized gains (losses)		0.0
Amortization/Accretion		0.0
Purchases		0.0
Sales		0.0
Transfers in		0.0
Transfers out		0.0
Ending balance		0.1
Other long-term investments \| Level 3 Investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	138.7	654.0	138.7
Net realized and unrealized gains (losses)		275.6	154.2
Amortization/Accretion		0.0	0.0
Purchases		78.8	211.3
Sales		(8.1)
Transfers in		0.0	82.6
Transfers out		0.0	0.0
Ending balance		1,000.3	586.8
Hedge and private equity funds included in other long-term investments \| NAV
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance	186.9	202.3	186.9
Net realized and unrealized gains (losses)		(14.9)	4.9
Amortization/Accretion		0.0	0.0
Purchases		39.8	106.4
Sales		(56.3)	(28.7)
Transfers in		0.0	0.0
Transfers out		0.0	(82.6)
Ending balance		170.9	186.9
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value		571.8	272.5	$ 201.2	$ 214.2
Realized and unrealized losses		$ 0.4	0.3
Asset Management (Kudu) \| NAV
Debt and Equity Securities, FV-NI [Line Items]
Purchases			70.1
Transfers out			$ (71.7)
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 114.7