Percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Variable interest rate in effect as of the balance sheet date related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeVariableInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of liability hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

+ Details

Name:	us-gaap_HedgedLiabilityFairValueHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: 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)(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 3: 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(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

+ Details

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

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=wtm_OtherOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=wtm_NSMBankFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=wtm_KuduBankFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wtm_KuduDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=wtm_TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wtm_OtherNSMMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=wtm_OtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=wtm_DelayedDrawTermLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=wtm_EmbracePetInsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_CreditRatingStandardPoorsAxis=srt_StandardPoorsAAARatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CreditRatingStandardPoorsAxis=srt_StandardPoorsAARatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CreditRatingStandardPoorsAxis=srt_StandardPoorsARatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CreditRatingStandardPoorsAxis=srt_StandardPoorsBBBRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CreditRatingStandardPoorsAxis=srt_StandardPoorsBBRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Narrative (Details) - USD ($) $ in Millions		3 Months Ended		8 Months Ended
Debt - Narrative (Details) - USD ($) $ in Millions	Apr. 01, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 23, 2019	Jun. 15, 2018	May 11, 2018
Debt Instrument
Fair value investments		$ 2,541.6	$ 2,534.6	$ 2,328.7	$ 2,746.0
Total debt		294.7			283.5
Repayments of debt		1.0	14.8
Other Operations
Debt Instrument
Total commitment under revolving credit facility		16.3
NSM Holding Co LLC
Debt Instrument
Total debt		218.4			219.2
NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt		217.1			217.4
Face amount								$ 100.0
Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility						$ 125.0
Kudu \| Kudu Debt [Member]
Debt Instrument
Total debt		65.7			53.6
Term Loan \| Other Operations
Debt Instrument
Total commitment under revolving credit facility		11.3
Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt		220.7
Face amount		$ 220.7			$ 221.3			224.0
Effective percentage		6.80%			7.50%
Unamortized issuance cost		$ (3.6)			$ (3.9)
Repayments of debt		0.6	0.5
Proceeds from borrowings			$ 6.5
Term Loan \| NSM Holding Co LLC \| Other NSM
Debt Instrument
Total debt		1.3			$ 1.8
Face amount		$ 1.6
Effective percentage		2.20%			3.00%
Term Loan \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Face amount		$ 69.0			$ 57.0
Total commitment under revolving credit facility						57.0
Proceeds from Issuance of Long-term Debt		$ 12.0
Term Loan \| Kudu \| Kudu Debt [Member]
Debt Instrument
Effective percentage		9.10%			8.30%
Unamortized issuance cost		$ (3.3)			$ (3.4)
Term Loan \| Other \| Other
Debt Instrument
Total debt		10.6			10.7
Face amount		$ 11.0			$ 11.1
Effective percentage		8.20%			8.30%
Unamortized issuance cost		$ (0.4)			$ (0.4)
Delayed Draw Term Loans [Member] \| Other Operations
Debt Instrument
Total commitment under revolving credit facility		3.0
Delayed Draw Term Loans [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility						63.0
Delayed Draw Term Loans [Member] \| Kudu \| Kudu Debt [Member]
Debt Instrument
Total commitment under revolving credit facility					125.0
Revolving Credit Facility \| Other Operations
Debt Instrument
Total commitment under revolving credit facility		$ 2.0
Revolving Credit Facility \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total commitment under revolving credit facility								10.0
Borrowings under facility								$ 2.0
Revolving Credit Facility \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility						$ 5.0
Interest Rate Swap \| NSM Holding Co LLC
Debt Instrument
Derivative, fixed interest rate							2.97%
Derivative, variable interest rate		1.58%
Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt		$ 148.4
Total commitment under revolving credit facility							$ 151.0
Hedged Liability, Fair Value Hedge							$ 151.0
Derivative, fixed interest rate		2.97%
Debt, weighted average interest rate, all-in		6.34%
London Interbank Offered Rate (LIBOR) \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate		1.00%
London Interbank Offered Rate (LIBOR) \| Minimum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		4.25%		4.25%
London Interbank Offered Rate (LIBOR) \| Minimum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate		5.50%
London Interbank Offered Rate (LIBOR) \| Maximum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		4.75%		4.75%
London Interbank Offered Rate (LIBOR) \| Maximum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate		6.25%
Prime Rate \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate		1.00%
Prime Rate \| Minimum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		3.25%		3.25%
Prime Rate \| Minimum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate		4.50%
Prime Rate \| Maximum \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Basis spread on variable rate		3.75%		3.75%
Prime Rate \| Maximum \| Line of Credit [Member] \| Kudu \| Kudu Bank Facility [Member]
Debt Instrument
Basis spread on variable rate		5.25%
Designated as Hedging Instrument \| Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt		$ 148.4
Debt, weighted average interest rate, all-in		7.47%
Not Designated as Hedging Instrument \| Interest Rate Swap \| Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt		$ 72.3
Effective percentage		6.25%
Embrace \| NSM Holding Co LLC
Debt Instrument
Proceeds from borrowings	$ 20.4
Carrying Value \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Total debt		$ 217.1			217.4
Debt securities issued by corporations
Debt Instrument
Fair value investments		486.7			467.2
Debt securities issued by corporations \| AAA
Debt Instrument
Fair value investments		11.3			9.5
Debt securities issued by corporations \| AA
Debt Instrument
Fair value investments		63.6			73.9
Debt securities issued by corporations \| Standard & Poor's, A Rating [Member]
Debt Instrument
Fair value investments		304.7			288.5
Debt securities issued by corporations \| BBB
Debt Instrument
Fair value investments		106.1			95.3
Debt securities issued by corporations \| BB
Debt Instrument
Fair value investments		$ 1.0			$ 0.0