Amount of increase (decrease) in debt instruments, classified as other.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

+ Details

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

- 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=120520924&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=120400017&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=120398452&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=120391452&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=121586364&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 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=121586364&loc=d3e3291-108585

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=wtm_TermLoanMember
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_NSMInsuranceGroupMember
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_BusinessAcquisitionAxis=wtm_KingsbridgeMember
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_BusinessAcquisitionAxis=wtm_RenewalRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Schedule of the Change in Debt under the NSM Bank Facility (Details) £ in Millions, $ in Millions				3 Months Ended		9 Months Ended
	Apr. 07, 2020 GBP (£)	Apr. 07, 2020 USD ($)	Apr. 01, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 GBP (£)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Debt Instrument
Beginning balance							$ 283.5
Repayments of debt							(2.7)	$ (22.2)
Ending balance				$ 359.4			359.4
Term Loan \| NSM Bank Facility
Debt Instrument
Beginning balance							217.4
Ending balance				268.8			268.8
Specialty Insurance Distribution (NSM)
Debt Instrument
Beginning balance							219.2
Ending balance				270.2			270.2
Specialty Insurance Distribution (NSM) \| NSM Bank Facility
Debt Instrument
Beginning balance				273.4	$ 222.3		221.3	180.4
Ending balance				275.0	221.9		275.0	221.9
Specialty Insurance Distribution (NSM) \| Term Loan \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings	£ 42.5	$ 52.4		0.0	0.0	£ 42.5	52.4	42.9
Repayments of debt				(0.7)	(0.4)		(1.3)	(1.4)
Foreign currency translation				2.3	0.0		2.6	0.0
Specialty Insurance Distribution (NSM) \| Revolving credit loan \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings				0.0	0.0		0.0	6.5
Repayments of debt				$ 0.0	$ 0.0		0.0	(6.5)
Kingsbridge Group Limited \| Specialty Insurance Distribution (NSM)
Debt Instrument
Proceeds from borrowings	£ 42.5	$ 52.4
Kingsbridge Group Limited \| Specialty Insurance Distribution (NSM) \| Term Loan \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings							$ 52.4
Embrace Pet Insurance \| Specialty Insurance Distribution (NSM)
Debt Instrument
Proceeds from borrowings			$ 20.4
Embrace Pet Insurance \| Specialty Insurance Distribution (NSM) \| Term Loan \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings								20.4
Renewal rights \| Specialty Insurance Distribution (NSM) \| Term Loan \| NSM Bank Facility
Debt Instrument
Proceeds from borrowings								$ 22.5