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

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- 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
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types: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

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
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-SubTopic 10
-Section 45
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-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

+ Details

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

- 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Debt Covenant, Loan to Value, Maximum Percent

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Covenant, Loan to Value, Maximum Percent, Period Five

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
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Balance Type:	na
Period Type:	instant

- Definition

Debt Covenant, Loan to Value, Maximum Percent, Period Four

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Covenant, Loan to Value, Maximum Percent, Period One

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Covenant, Loan to Value, Maximum Percent, Period Three

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Covenant, Loan to Value, Maximum Percent, Period Two

+ References

No definition available.

+ Details

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Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Interest Reserve

+ References

No definition available.

+ Details

Name:	wtm_InterestReserve
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage of borrowing base

+ References

No definition available.

+ Details

Name:	wtm_PercentageOfBorrowingBase
Namespace Prefix:	wtm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=wtm_KuduBankFacilityMember
Namespace Prefix:
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Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_RevolvingCreditFacilityMember
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Balance Type:
Period Type:

- Details

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- Details

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Namespace Prefix:
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- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=wtm_TermLoanMember
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Debt - Kudu Credit and Bank Facility (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Mar. 23, 2021
Debt Instrument
Debt covenant, maximum loan to value percent, period one	50.00%
Debt covenant, maximum loan to value percent, period two	40.00%
Debt covenant, maximum loan to value percent, period three	25.00%
Debt covenant, maximum loan to value percent, period four	15.00%
Debt covenant, maximum loan to value percent, period five	0.00%
Debt covenant, maximum loan to value percent	33.00%
Kudu Bank Facility \| Revolving Credit Facility
Debt Instrument
Effective yield (as a percent)	0.25%
Kudu Bank Facility \| Revolving Credit Facility \| London Interbank Offered Rate (LIBOR)
Debt Instrument
Basis spread on variable rate	4.30%
Kudu Bank Facility \| Kudu
Debt Instrument
Total commitment under revolving credit facility			$ 300,000,000
Kudu Bank Facility \| Kudu \| Revolving Credit Facility
Debt Instrument
Loss on debt extinguishment		$ 4,100,000
Kudu Bank Facility \| Kudu \| Line of Credit
Debt Instrument
Interest reserve	$ 12,200,000	$ 4,500,000
Kudu Bank Facility \| Kudu \| Term Loan
Debt Instrument
Percentage of borrowing base	35.00%
Undrawn amount on facility	$ 45,900,000